Chat Generative Pre-trained Transformer: why we should embrace this technology.

With the advent of artificial intelligence that not only can learn from us but also can communicate with us in plain language, humans are embarking on a brave new future. The interaction between humans and artificial intelligence has never been so widespread. Chat Generative Pre-trained Transformer is an artificial intelligence resource that has potential uses in the practice of medicine. As clinicians, we have the opportunity to help guide and develop new ways to use this powerful tool. Optimal use of any tool requires a certain level of comfort. This is best achieved by appreciating its power and limitations. Being part of the process is crucial in maximizing its use in our field. This clinical opinion demonstrates the potential uses of Chat Generative Pre-trained Transformer for obstetrician-gynecologists and encourages readers to serve as the driving force behind this resource.

Use of cervical elastography at 18 to 22 weeks' gestation in the prediction of spontaneous preterm birth.

BACKGROUND: Accurate identification of the women who will have spontaneous preterm birth continues to be a great challenge. The use of cervical elastography for prediction of preterm birth is promising, but several limitations exist. Newer cervical elastography technology has been developed that may prove useful in evaluation of risk of preterm birth. OBJECTIVE: This study aimed to develop standard cervical elastography nomograms for singleton pregnancies at 18 to 22 weeks' gestation using the E-Cervix ultrasound application, assess intraobserver reliability of the E-Cervix elastography parameters, and determine whether these cervical elastography measurements can be used in the prediction of spontaneous preterm birth. STUDY DESIGN: This was a prospective cohort study of pregnant women undergoing cervical length screening assessment via transvaginal ultrasound examination at 18 to 22 weeks' gestation. A semiautomatic, cervical elastography application (E-Cervix) was used during the transvaginal examination to calculate 5 quantitative parameters (internal os stiffness, external os stiffness, internal -to -external os stiffness ratio, hardness ratio, and elasticity contrast index) and create a standard nomogram for each one of them. The intraobserver reliability was calculated using Shrout-Fleiss reliability. Cervical elastography parameters were compared between those who delivered preterm (<37 weeks) spontaneously and those who delivered full term. A multivariable logistic regression model was performed to determine the ability of the cervical elastography parameters to predict spontaneous preterm birth. RESULTS: A total of 742 women were included, of which 49 (6.6%) had a spontaneous preterm delivery. A standard nomogram was created for each of the cervical elastography parameters from those who had a full-term birth in the index pregnancy (n=693). Intraobserver reliability was good or excellent (intraclass correlation, 0.757-0.887) for each of the cervical elastography parameters except external os stiffness which was poor (intraclass correlation, 0.441). In univariate analysis, none of the cervical elastography parameters were associated with a statistically significant increased risk of spontaneous preterm birth. In a multivariable model adjusting for history of preterm birth, gravidity, ethnicity, cervical cerclage, and vaginal progesterone use, increasing elasticity contrast index was significantly associated with an increased risk of spontaneous preterm birth (odds ratio, 1.15; 95% confidence interval, 1.02-1.30; P=.02). CONCLUSION: Cervical elastography parameters are reliably measured and are stable across 18 to 22 weeks' gestation. Based on our findings, the elasticity contrast index was associated with an increased risk of spontaneous preterm birth and may be a useful parameter for future research.

Experiences of early graduate medical students working in New York hospitals during the COVID-19 pandemic: a mixed methods study.

BACKGROUND: The coronavirus disease 2019 (COVID-19) pandemic presented the world with a sudden need for additional medical professionals. Senior medical students were identified as potential workers and many worldwide graduated early to serve as Junior Physicians in hospitals. The authors sought to identify factors that informed the decision to work, describe experiences in this capacity, and elucidate benefits for trainees. METHODS: The investigators conducted a mixed-methods observational cohort study of early medical graduates eligible to work as Junior Physicians at two New York medical centers in April/May 2020 during an initial surge in COVID-19 hospitalizations. Graduates were surveyed, and a sample of Junior Physicians participated in a focus group. Survey responses of those who worked were compared to those who did not. Focus group responses were transcribed, coded, and thematically analyzed. RESULTS: Fifty-nine graduates completed the study methods and 39 worked as Junior Physicians. Primary reasons for working included duty to help (39 [100%]), financial incentive (32 [82%]), desire to learn about pandemic response (25 [64%]), and educational incentive (24 [62%]). All had direct contact with COVID-19 patients, believed working was beneficial to their medical training, and were glad they worked. None contracted a symptomatic infection while working. Compared with non-Junior Physicians, Junior Physicians reported increased comfort levels in completing medical intern-level actions like transitions of care functions, such as writing transfer notes (P < 0.01), writing discharge orders (P = 0.01), and providing verbal sign out (P = 0.05), and they reported more comfort in managing COVID-19 patients. Sixteen themes emerged from the focus group and were placed into four categories: development of skills, patient care, safety, and wellness. CONCLUSIONS: Senior medical students chose to work as Junior Physicians for both personal and educational reasons. Experiences were beneficial to trainees and can inform future innovations in medical education.

Term cesarean delivery in the first pregnancy is not associated with an increased risk for preterm delivery in the subsequent pregnancy.

BACKGROUND: Prior studies have reported an increased risk for preterm delivery following a term cesarean delivery. However, these studies did not adjust for high-risk conditions related to the first cesarean delivery and are known to recur. OBJECTIVE: The objective of the study was to determine whether there is an association between term cesarean delivery in the first pregnancy and subsequent spontaneous or indicated preterm delivery. STUDY DESIGN: This was a retrospective cohort study of women with the first 2 consecutive singleton deliveries (2007-2014) identified through a linked pregnancy database at a single institution. Women with a first pregnancy that resulted in cesarean delivery at term were compared with women whose first pregnancy resulted in a vaginal delivery at term. Exclusion criteria were known to recur medical or obstetrical complications during the first pregnancy. A propensity score analysis was performed by matching women who underwent a cesarean delivery with those who underwent a vaginal delivery in the first pregnancy. The association between cesarean delivery in the first pregnancy and preterm delivery in the second pregnancy in this matched set was examined using conditional logistic regression. The primary outcome was overall preterm delivery <37 weeks in the second pregnancy. Secondary outcomes included type of preterm delivery (spontaneous vs indicated), late preterm delivery (34-36 6/7 weeks), early preterm delivery (<34 weeks), and small-for-gestational-age birth. RESULTS: Of a total of 6456 linked pregnancies, 2284 deliveries were matched; 1142 were preceded by cesarean delivery and 1142 were preceded by vaginal delivery. The main indications for cesarean delivery in the first pregnancy were dystocia in 703 (61.5%), nonreassuring fetal status in 222 (19.4%), breech presentation in 100 (8.8%), and other in 84 (7.4%). The mean (SD) gestational ages at delivery for the second pregnancy was 38.8 (1.8) and 38.9 (1.7) weeks, respectively, for prior cesarean delivery and vaginal delivery. The risks of preterm delivery in the second pregnancy among women with a previous cesarean and vaginal delivery were 6.0% and 5.2%, respectively (adjusted odds ratio, 1.46, 95% confidence interval, [CI] 0.77-2.76). In an analysis stratified by the type of preterm delivery in the second pregnancy, no associations were seen between cesarean delivery in the first pregnancy and spontaneous preterm delivery (4.6% vs 3.9%; adjusted odds ratio, 1.40, 95% confidence interval, 0.59-3.32) or indicated preterm delivery (1.6% vs 1.4%; adjusted odds ratio, 1.21, 95% confidence interval, 0.60-2.46). Similarly, no significant differences were found in late preterm delivery (4.6% vs 4.1%; adjusted odds ratio, 1.13, 95% confidence interval, 0.55-2.29), early preterm delivery (1.6% vs 1.2%; adjusted odds ratio, 1.25, 95% confidence interval, 0.59-2.67), or neonates with birthweight less than the fifth percentile for gestational age (3.6% vs 2.2%; adjusted odds ratio, 1.26, 95% confidence interval, 0.52-3.06). CONCLUSION: After robust adjustment for confounders through a propensity score analysis related to the indication for the first cesarean delivery at term, cesarean delivery is not associated with an increase in preterm delivery, spontaneous or indicated, in the subsequent pregnancy.

The use of cervical sonography to differentiate true from false labor in term patients presenting for labor check.

BACKGROUND: Cervical length by transvaginal ultrasound to predict preterm labor is widely used in clinical practice. Virtually no data exist on cervical length measurement to differentiate true from false labor in term patients who present for labor check. False-positive diagnosis of true labor at term may lead to unnecessary hospital admissions, obstetrical interventions, resource utilization, and cost. OBJECTIVE: We sought to determine if cervical length by transvaginal ultrasound can differentiate true from false labor in term patients presenting for labor check. STUDY DESIGN: This is a prospective observational study of women presenting to labor and delivery with labor symptoms at 37-42 weeks, singleton cephalic gestation, regular uterine contractions (≥4/20 min), intact membranes, and cervix ≤4 cm dilated and ≤80% effaced. Those patients with placenta previa and indications for immediate delivery were excluded. The shortest best cervical length of 3 collected images was used for analysis. Providers managing labor were blinded to the cervical length. True labor was defined as spontaneous rupture of membranes or spontaneous cervical dilation ≥4 cm and ≥80% effaced within 24 hours of cervical length measurement. In the absence of these outcomes, labor status was determined as false labor. Receiver operating characteristic curves were generated to assess the predictive ability of cervical length to differentiate true from false labor and were analyzed separately for primiparous and multiparous patients. The diagnostic accuracies of various cervical length cutoffs were determined. The relationship of cervical length and time to delivery was also analyzed including both use and nonuse of oxytocin. RESULTS: In all, 77 patients were included in the study; the prevalence of true labor was 58.4% (45/77). Patients who were in true labor had shorter cervical length as compared to those in false labor: median 1.3 cm (range 0.5-4.1) vs 2.4 cm (range 1.0-5.0), respectively (P < .001). The area under the receiver operating characteristic curve for primiparous patients was 0.88 (P < .001) and for multiparous patients was 0.76 (P < .01), both demonstrating good correlation. The area under the receiver operating characteristic curves were not significantly different between primiparous and multiparous (P = .23). The area under the receiver operating characteristic curve for primiparous and multiparous patients combined was 0.8 (P < .0001), indicating a good overall correlation between cervical length and its ability to differentiate true from false labor. Overall, a cervical length cutoff of ≤1.5 cm to predict true labor had the highest specificity (81%), positive predictive value (83%), and positive likelihood ratio (4.2). There were no differences in cervical length prediction between primiparous and multiparous patients. Cervical length was positively correlated with time to delivery, regardless of the use of oxytocin. CONCLUSION: In differentiating true from false labor in term patients who present for labor check, a cervical length of ≤1.5 cm was the most clinically optimal cutoff with the lowest false positive rate-due to its highest specificity-and highest positive predictive value and positive likelihood ratios. Its use to decide admission in patients at term with labor symptoms may prevent unnecessary admissions, obstetrical interventions, resource utilization, and cost.

The timing of administration of antenatal corticosteroids in women with indicated preterm birth.

OBJECTIVE: We sought to determine the timing of administration of antenatal corticosteroids (AS) for indicated preterm births and to identify which indications are associated with the most optimal timing of administration. STUDY DESIGN: This was a retrospective cohort of patients who received AS in anticipation of indicated preterm birth from 2009 through 2012 at Winthrop University Hospital, Mineola, NY. Medical records of patients who received AS, as identified through the hospital pharmacy database, were reviewed. Patients were included if they had a singleton or twin gestation and they received AS for maternal or fetal indications. Women were excluded if they received AS for spontaneous preterm labor or preterm rupture of membranes. Maternal demographic and obstetrical characteristics were compared between those who received AS≤7 days vs >7 days from delivery using parametric and nonparametric tests with relative risks and 95% confidence intervals. P<.05 was considered significant. RESULTS: In all, 193 patients were included in this study. Median latency from AS administration to delivery was 9 days (range, 0-83); 93 patients (48%) received AS within 7 days of delivery. There were no significant differences between the 2 groups with regards to baseline maternal characteristics. Those delivering within 7 days of AS administration were more likely to have maternal vs fetal indications (84% vs 16%). CONCLUSION: Only 48% of patients with an indication for preterm birth received AS within 7 days of its administration. AS appear to be more optimally timed in the presence of maternal rather than fetal indications.

Reduced folate carrier 80A-->G polymorphism, plasma folate, and risk of placental abruption.

Folate deficiency and maternal smoking are strong risk factors for placental abruption. We assessed whether the reduced folate carrier [NM_194255.1: c.80A-->G (i.e., p.His27Arg)] (RFC-1) polymorphism was associated with placental abruption, and evaluated if maternal smoking modified the association between plasma folate and abruption. Data were derived from the New Jersey-Placental Abruption Study--a multicenter, case-control study of placental abruption (2002-2007). Maternal DNA was assayed for the RFC-1 c.80A-->G polymorphism using a PCR-dependent diagnostic test. Maternal folate (nmol/l) was assessed from maternal plasma, collected immediately following delivery. Due to assay limitations, folate levels at > or =60 nmol/l were truncated at 60 nmol/l. Therefore, case-control differences in folate were assessed from censored log-normal regression models following adjustment for potential confounders. Distribution of the mutant allele (G) of the RFC-1 c.80A-->G polymorphism was similar between cases (52.3%; n = 196) and controls (50.5%; n = 191), as was the homozygous mutant (G/G) genotype (OR 1.1, 95% CI 0.6-2.2). In a sub-sample of 136 cases and 140 controls, maternal plasma folate levels (mean +/- standard error) corrected for assay detection limits were similar between placental abruption cases (63.6 +/- 5.1 nmol/l) and controls (58.3 +/- 4.7 nmol/l; P = 0.270), and maternal smoking did not modify this relationship (interaction P = 0.169). We did not detect any association between the RFC-1 c.80A-->G polymorphism and placental abruption, nor was an association between plasma folate and abruption risk evident. These findings may be the consequence of high prevalence of prenatal multivitamin and folate supplementation in this population (over 80%). It is therefore not surprising that folate deficiency may be rare and that the RFC-1 c.80A-->G polymorphism is less biologically significant for placental abruption.

Risk factors for antepartum and intrapartum stillbirth: a population-based study.

To examine disparities in risk factors for stillbirths and its occurrence in the antepartum versus intrapartum periods. A population-based, cross-sectional study using data on women that delivered singleton births between 20 and 43 weeks in Missouri (1989-1997) was conducted (n = 626,883). Hazard ratios and 95% confidence intervals were derived from regression models and population attributable fractions were estimated to examine the impact of risk factors on stillbirth. Among African Americans, risks of antepartum and intrapartum stillbirth were 5.6 and 1.1 per 1,000 singleton births, respectively; risks among whites were 3.4 and 0.5 per 1,000 births, respectively. Maternal age > or = 35 years, lack of prenatal care, prepregnancy body mass index (BMI) > or = 30 kg/m2, and prior preterm or small-for-gestational age birth were significantly associated with increased risk for antepartum stillbirth among whites, but not African Americans. BMI < or = 18.5 kg/m2 was associated with antepartum and intrapartum stillbirth among African Americans, but not whites. The presence of any congenital anomaly, abruption, and cord complications were associated with antepartum stillbirth in both races. Premature rupture of membranes was associated with intrapartum stillbirth among whites and African Americans, but intrapartum fever was associated with intrapartum stillbirth among African Americans. These risk factors were implicated in 54.9% and 19.7% of antepartum and intrapartum stillbirths, respectively, among African American women, and in a respective 46.6% and 11.9% among white women. Considerable heterogeneity in risk factors between antepartum and intrapartum stillbirths is evident. Knowledge on timing of stillbirth specific risk factors may help clinicians in decreasing antepartum and intrapartum stillbirth risks through monitoring and timely intervention.

Chronic hypertension and risk of placental abruption: is the association modified by ischemic placental disease?

OBJECTIVE: The purpose of this study was to evaluate whether the increased risk of placental abruption among women with chronic hypertension is modified by ischemic placental disease, specifically pregnancy-induced hypertension (PIH) and fetal growth restriction (FGR). STUDY DESIGN: We used the US linked natality and fetal death data files (1995-2002) and restricted the analysis to women who had a singleton birth at > or = 22 weeks of gestation and to fetuses who weighed > or = 500 g (n = 30,189,949). Fetal growth was defined both on a continuum (<1, 1-2, 3-4, 5-9, 10-19, ..., > or = 90) and as birthweight of < 10th percentile for gestational age (FGR) or birthweight of > 90th percentile (large for gestational age [LGA]). All analyses were adjusted for potential confounding factors through multivariable logistic regression. RESULTS: Rates of abruption among women with and without chronic hypertension were 15.6 and 5.8 per 1000 pregnancies, respectively (relative risk [RR], 2.4; 95% CI, 2.3, 2.5). In comparison with normotensive women with appropriately grown babies (ie, 10th-90th percentile), the association between chronic hypertension and abruption was modified in the presence of FGR (RR, 3.8; 95% CI, 3.6, 4.1) and PIH (RR, 7.7; 95% CI, 6.6, 8.9). However, the highest risk was seen among women with chronic hypertension, PIH, and LGA (RR, 9.0; 95% CI, 7.2, 11.3). A dose-response relationship was observed between the risk of abruption and fetal growth (assessed on a continuum), with the risk being lowest among LGA babies. CONCLUSION: The association between chronic hypertension and abruption is strong; ischemic placental disease (PIH and FGR) modified this relationship. These findings suggest an etiologic relationship between abruption and chronic placental disease. Chronic hypertension, if associated with LGA, is not associated with abruption; however, chronic hypertension with superimposed PIH accompanied by LGA is associated with significantly increased risk.

Fetal growth restriction and subsequent pregnancy risks.

Fetal growth restriction can result from a variety of intrinsic or extrinsic insults, resulting from maternal, fetal, and placental factors. Determining the underlying cause of poor fetal growth can be difficult but is essential for assessing potential risks for future pregnancies. Importantly, recurrence risks greatly depend on these underlying conditions. Understanding these risks can allow more appropriate patient counseling and may influence management strategies to optimize future pregnancies.

Does educational intervention affect resident competence in sonographic cervical length measurement?

OBJECTIVE: To determine if a structured teaching module improves resident competency in transvaginal sonographic cervical length measurements. METHODS: This was a prospective cohort study involving obstetrics and gynecology residents at a single institution. Residents collected 10 transvaginal cervical images from patients with threatened preterm labor presenting to Labor and Delivery. After initial image acquisition, residents participated in a lecture-based teaching module involving a pre- and post-intervention assessment. Following the didactic session, they collected 10 additional images. All the images were scored independently by two Maternal-Fetal Medicine attending physicians based on the quality and accuracy of the measured cervical length. Pre-and post- intervention test results were compared, as well as pre- and post- intervention image scores. Parametric and nonparametric tests were used as appropriate with p < 0.05 considered significant. RESULTS: Ninety-three percent of the residents (14/15) improved their scores from pre-test to post-test or maintained an already perfect score (p < 0.01). Improvement was most significant with the junior residents. Seventy-nine percent of the residents (11/14) improved their cervical image scores after the educational session. Mean score for total residents was 73.7 + 12.6 pre-intervention and 90.2 + 9.9 post-intervention (p < 0.01) out of a total of 120. CONCLUSIONS: There is an improvement in the competence of resident measured cervical lengths via transvaginal ultrasound when a structured educational module is implemented for resident education.

Practice patterns in the timing of antenatal corticosteroids for fetal lung maturity.

OBJECTIVE: To determine the practice patterns of antenatal corticosteroid (AS) administration in women with threatened preterm labor. METHODS: This was a retrospective cohort of patients who received betamethasone between 2009 and 2010, identified through a pharmacy database. Patients with high order multiples; incomplete records and indicated preterm delivery were excluded. Demographic and obstetrical factors were compared between women with an AS to delivery latency of ≤7 days versus >7 days. Parametric and non-parametric tests were used as appropriate. p < 0.05 denotes statistical significance; relative risks with 95% confidence intervals were calculated. RESULTS: Three-hundred forty-five patients were included. Sixty-eight patients (20%) received AS within 7 days of delivery. Women who received AS ≤7 days before delivery (optimal timing) were more likely to have a transvaginal cervical length ≤2 cm (RR:2.53, CI: 1.2-5.6), cervical dilation ≥2 cm (RR: 3.86, CI: 2.7-5.6) and positive fFN (RR: 2.59, CI: 1.1-6.3). Preterm premature ruptured membranes were also associated with optimal timing of AS (RR: 4.86, CI: 3.4-6.8). CONCLUSIONS: Eighty percent of patients receive suboptimal timing of AS administration. Factors associated with suboptimal timing are: cervical length >2 cm, cervical dilation <2 cm and negative fFN. Cervical assessment should be a key factor in the decision for AS administration. More research is needed for accurate timing of AS in women with threatened preterm labor.

Perinatal mortality in first- and second-born twins in the United States.

OBJECTIVE: To evaluate the prevailing mortality paradox that second-born twins are at higher risk of perinatal mortality than first-born twins. METHODS: We used the 1995-1997 United States "matched multiple birth" data files assembled by the National Center for Health Statistics, for analysis of risk of perinatal mortality in first- and second-born twins (293788 fetuses). Perinatal mortality was defined to include stillbirths after 20 weeks of gestation and neonatal deaths (deaths within the first 28 days). Gestational age-specific risk of perinatal mortality (per 1000 total births), stillbirth (per 1000 total births), and neonatal mortality (per 1000 livebirths) by order of twin birth were based on the fetuses-at-risk approach. Associations between order of birth and mortality indices were evaluated by fitting multivariable logistic regression models based on the method of generalized estimating equations. These models were adjusted for several potential confounding factors. RESULTS: Perinatal mortality was 37% higher in second-born (26.1 per 1000 total births) than in first-born (20.3 per 1000 total births) twins (adjusted relative risk [RR] 1.37; 95% confidence interval [CI] 1.32, 1.42). The increased risk of perinatal mortality in second-born twins was chiefly driven by a 2.46-fold (95% CI 2.29, 2.63) increase in the number of stillbirths. However, the risk of neonatal mortality was very similar between first- and second-born twins (RR 0.99, 95% CI 0.95, 1.04). CONCLUSIONS: The increased risk of perinatal death in second-born twins is driven chiefly by increased rates of stillborn second twins. Thus, the increased mortality in second-born over first-born twins probably is an artifact of mortality comparisons.

Birth weight discordance and adverse fetal and neonatal outcomes among triplets in the United States.

OBJECTIVE: To examine the association between intratriplet birth weight discordance, fetal and neonatal mortality, and smallness for gestational age. METHODS: The 1995-1997 Centers for Disease Control and Prevention's Matched Multiple Birth file was used for this analysis. Birth weight discordance was calculated as the difference in birth weight between the largest and the smallest triplet's weight and expressed as percentage of the largest triplet's weight. For the middle-weight triplet, we also used the largest triplet's weight as a reference in calculating percentage birth weight discordance, which was then grouped into quintiles. RESULTS: Among 15,511 triplet live births and fetal deaths (at least 20 weeks' gestation), 35% had less than 10% birth weight discordance, 19.3% had 10-15%, 16.4% had 15-21%, 15.2% had 21-29%, and 14.1% had 29% or more. After controlling for confounders, the risk of fetal death associated with quintile V was significantly higher than that associated with quintile I for smallest (odds ratio [OR] 10.88; 95% confidence interval [CI] 4.87, 26.56), middle (OR 22.6; 95% CI 11.05, 46.3), and largest (OR 2.41; 95% CI 1.01, 5.89) triplets. Smallest and middle triplets in quintile V were more likely than quintile I triplets to be born small for gestational age (OR 26.0; 95% CI 17.1, 39.9 for smallest, and OR 13.4; 95% CI 8.01, 22.3 for middle). Birth weight discordance quintile was not associated with smallness for geatational age among largest triplets nor consistently with neonatal mortality among smallest, middle, or largest triplets. CONCLUSION: Increasing birth weight discordance was associated with increased risk of fetal death and smallness for gestational age. A birth weight discordance threshold of at least 29% should alert obstetricians for appropriate decision making.

Twin deliveries in the United States over three decades: an age-period-cohort analysis.

OBJECTIVE: Time is an important variable in understanding the recent increase in twin deliveries in the United States. Therefore, this study was designed to estimate the influences of maternal age, period (year) of delivery, and maternal-birth-year cohort on trends in rates of twin deliveries. METHODS: United States natality data were used to assess trends in twin pregnancies resulting in live births. This age-period-cohort analysis included 7, 5-year maternal-age groups (15-19 through 45-49 years), 6 twin delivery periods (1975, 1980, 1985, 1990, 1995, and 2000), and 12, 5-year maternal birth cohorts (1926-1930 through 1981-1985). The independent effects of maternal age, twin delivery period, and maternal birth cohort on twin delivery rates for blacks and whites were modeled using Poisson regression techniques. RESULTS: Our study assessed 95,042 blacks and 401,989 whites with twin deliveries. Twin deliveries increased by 46% for blacks and 62% for whites from 1975 to 2000, with the largest increase occurring in the year 2000. For blacks, maternal age had the strongest impact on the increasing twin delivery rates, followed by period of delivery. For whites, the greatest effect was due to period of delivery, followed by maternal birth year cohort and, lastly, maternal age. CONCLUSION: Our data confirm the importance of nature's biologic contribution of maternal aging to twin delivery rates, but suggest that recent changes in the environment surrounding pregnancy (nurture) also influence twin delivery rates. The relative contributions of biologic versus environmental influences appear to differ among blacks and whites.

Noninvasive ultrasound assessment of maternal vascular reactivity during pregnancy: a longitudinal study.

OBJECTIVE: To estimate the pattern of maternal vascular reactivity in normal and high-risk pregnancies using postocclusion brachial artery diameter. METHODS: Prospective, longitudinal study of 44 low-risk singleton pregnancies and 28 high-risk pregnancies, defined as pregestational diabetes (n = 7), chronic hypertension (n = 4), twin gestation (n = 6), and a previous history of preeclampsia, fetal growth restriction, or vascular disease (n = 11). During each trimester, the brachial artery was ultrasonographically imaged above the antecubital crease. Brachial artery diameter was measured and then occluded for 5 minutes using an inflated blood pressure cuff. Changes in brachial artery diameter at 1 minute after occlusion were expressed as percent change from baseline and were compared across trimesters for both low-risk and high-risk groups, adjusting for potential confounders. RESULTS: Brachial artery diameters were increased after occlusion in every trimester for all groups. For low-risk women, the degree of postocclusion brachial artery dilatation was similar in the first and second trimesters, but was lower in the third trimester. In the first trimester, low-risk women had significantly greater brachial artery diameter increases at 1 minute compared with high-risk singleton pregnancies (19% compared with 12%; P <.001). Compared with low-risk women, pregnancies complicated by pregestational diabetes or chronic hypertension had significantly smaller 1-minute brachial artery diameter changes in the first trimester (7.0 +/- 0.5%, P <.001), whereas twin gestations had greater brachial artery responses (22.9 +/- 6.0%, P <.001). Women with previous preeclampsia or vascular disease had responses similar to low-risk women. CONCLUSION: Maternal vascular reactivity as assessed by postocclusion brachial artery dilatation decreases in the third trimester in both low-risk and high-risk women. In addition, singleton pregnancies at high risk for preeclampsia display decreased brachial artery reactivity compared with low-risk women.