Predictive capacity of fetal pancreatic circumference for gestational diabetes mellitus.

OBJECTIVE: To assess the capacity of fetal pancreatic size, before standard blood testing for screening and diagnosis, to predict maternal gestational diabetes mellitus (GDM). METHODS: This was a retrospective cohort study of low-risk pregnant women recruited during routine second-trimester fetal anatomical screening at 20-25 weeks' gestation at two ultrasound units in Israel between 2017 and 2020. The predictive performance of fetal pancreatic circumference ≥ 80th and ≥ 90th centiles and glucose challenge test (GCT) was examined for the outcome of GDM. The independent-samples t-test was used to compare mean pancreatic circumference centile between pregnancies with GDM and those without GDM. Diagnostic performance was evaluated with 2 × 2 contingency tables and receiver-operating-characteristics (ROC) curves. RESULTS: Overall, 195 women were selected for statistical analysis. Twenty-four (12.3%) women were diagnosed subsequently with GDM. The mean ± SD pancreatic circumference centile was significantly higher in the GDM group compared with the non-GDM group (82.4 ± 14.6 vs 62.8 ± 27.6; P < 0.001). The pancreatic circumference centile was correlated positively with the estimated fetal weight centile (Pearson's coefficient, 0.243; P = 0.001). The 80th centile cut-off for pancreatic circumference had the highest sensitivity (70.8%) and positive predictive value (23.3%) for future maternal GDM, with the best trade-off between sensitivity and specificity achieved at the 75th centile cut-off (sensitivity, 79%; specificity, 60%). The GCT had better specificity (90.2%) and negative predictive value (97.9%) compared with both cut-offs in pancreatic circumference. The area under the ROC curve was higher for pancreatic circumference compared with GCT (0.71 vs 0.64) and only the former was statistically significant (P = 0.001). CONCLUSIONS: Fetal pancreatic circumference has a higher positive predictive capacity compared with GCT. Measuring pancreatic circumference can identify pregnancies at high risk for maternal GDM, thereby promoting earlier diagnosis and treatment, decreasing the time period during which the fetus is exposed to high maternal glucose levels and improving infant outcome. © 2024 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Afferent venous perfusion of fetal liver: umbilical and portal blood-flow volumes in fetuses born small-for-gestational age.

OBJECTIVE: To quantify the dynamic changes in the afferent venous flow volume of the liver in low-risk pregnancies with fetuses born small-for-gestational age. METHODS: This was a prospective study of low-risk singleton pregnancies with estimated fetal weight (EFW) and birth weight ≤ 10th centile attending for a routine second- or third-trimester ultrasound examination. Their umbilical and portal blood-flow volumes were compared with those of a control group of fetuses born appropriate-for-gestational age from which normal reference ranges were constructed. Absolute and Z-score differences between the groups were assessed. RESULTS: In total, 133 fetuses were included in the study group and 362 in the control group. The mean umbilical blood-flow volume in the study group, both absolute and normalized per kg of EFW, was below that of the appropriate-for-gestational-age fetuses for most of the period of pregnancy studied (overall mean Z-score, -0.82 and -0.84, respectively). In contrast, the mean portal blood-flow volume, per kg of EFW, showed the opposite trend (overall mean Z-score, +0.86), reaching its maximum level (+1.43) in the late third trimester. This resulted in a steep decrease in the mean placental-to-portal-blood-flow volume ratio, from 14.4 at 24 weeks of gestation (above the 60th centile) to 4.7 at 38 weeks of gestation (15th centile), corresponding to Z-scores of +0.4 and -1.02, respectively. CONCLUSION: In fetuses born small-for-gestational age, the ratio of blood-flow volume in the umbilical vein to that in the portal vein decreases consistently during pregnancy, and to a greater extent compared with those born appropriate-for-gestational age, reaching a lower nadir in the third trimester. This additional redistribution of liver perfusion affects negatively fetal growth even in low-risk pregnancy, and should be taken into account when planning delivery. We suggest considering liver venous perfusion as an ancillary tool for monitoring small-for-gestational-age pregnancies. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

Early Fetal Corpus Callosum: Demonstrating Normal Growth and Detecting Pathologies in Early Pregnancy.

BACKGROUND AND PURPOSE: A malformed corpus callosum carries a risk for abnormal neurodevelopment. The advent of high-frequency transducers offers the opportunity to assess corpus callosum development in early pregnancy. The aim of the study was to construct a reference chart of the fetal corpus callosum length on ultrasound between 13 and 19 weeks of gestation and to prospectively examine growth patterns in pathologic cases. MATERIALS AND METHODS: We performed a prospective cross-sectional study between 2020 and 2022 in well-dated, low-risk, singleton pregnancies between 13 and 19 weeks of gestation. A standardized image was obtained in the midsagittal plane. Imaging criteria were used as a confirmation of the early corpus callosum. Measurements were taken by 4 trained sonographers. Intra- and interobserver variability was assessed. Corpus callosum length in centiles were calculated for each gestational week. RESULTS: One hundred eighty-seven fetuses were included in the study. All cases met inclusion criteria. At 13 weeks of gestation, the margins of the early corpus callosum were sufficiently clear to be measured in 80% (20/25) of fetuses. A cubic polynomial regression model best described the correlation between corpus length and gestational age. The correlation coefficient (r 2) was 0.929 (P < .001). Intra- and interobserver variability had high interclass correlation coefficients (>0.99). Presented is the earliest published case of agenesis of corpus callosum and a case of dysgenetic corpus callosum in Rubinstein-Taybi syndrome. CONCLUSIONS: Provided is a nomogram of the early fetal corpus callosum. Applying imaging criteria helped to identify a case of complete agenesis of the corpus callosum as early as 14 weeks.

Choriovitelline placenta: prenatal sonographic imaging and clinical characteristics.

OBJECTIVES: To describe the fetal sonographic characteristics, in-utero natural history and postnatal outcome of choriovitelline placenta, in which the fetal umbilical vein is replaced by the extra-embryonic vitelline circulation. METHODS: This was a retrospective study of pregnancies examined during the period 2010-2014. Fetuses which presented with sonographic criteria of a downward caudal course of an enlarged vein from the umbilical annulus to the hepatic hilum were followed prospectively. Two-dimensional and three-dimensional color Doppler with high-definition flow were used in order to investigate the extra- and intrahepatic venous system. Ultrasound images and volumes were stored digitally, clinical data were obtained from patients' medical files and telephone interviews were conducted regarding the course of the pregnancy, perinatal data and developmental milestones. RESULTS: Four cases were identified during the study period. The mean ± SD gestational age at diagnosis was 19.5 ± 4.3 (range, 13-23) weeks. The characteristic downward course of the persistent vitelline vein was associated with aneurysmal dilatation and anomalous anatomical configuration of the intrahepatic venous system. One case ended with antepartum death at 28 weeks. The mean gestational age at delivery was 34.6 (±5.0) weeks and the birth weight corresponded to the 57.2nd (± 16.8 SD) centile. In two cases, a thrombotic mass was detected in the portal venous system after birth. One necessitated antithrombotic treatment for 6 months; in the other case, spontaneous resolution occurred 7 days after birth. In the three surviving infants, the persistent extrahepatic vitelline vein regressed gradually within 6 months after birth. Neurodevelopment was normal at follow-up aged 1 year and 7 months, 3 years and 6 months and 5 years and 5 months. CONCLUSION: The main clinical importance of choriovitelline placentation derives from the possible formation of thrombus in the portal venous system. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Can Ultrasound Performed in Prolonged Second Stage of Labor Predict the Difficulty and Success Rates of Operative Vaginal Delivery?

OBJECTIVE: To evaluate different sonographic methods for the prediction of the difficulty and the success of operative vaginal delivery (OPD). MATERIALS AND METHODS: A prospective study was performed on 45 term singleton uncomplicated pregnancies with prolonged 2nd stage of delivery with cephalic presentation. Measurements of the fetal head, relations between the fetal head and maternal pelvic parameters during rest and during maternal pushing were taken using translabial ultrasound. RESULTS: 29 cases of OPD were successful and 4 cases failed ending in cesarean section. The passage of the biparietal diameter (BPD) of the infrapubic line (IPL) was statistically correlated with the success of OPD. Head station, passage of the BPD of the IPL, percentage of head after the IPL, circumference of head after IPL were all correlated with the difficulty of OPD. When the distance between the widest diameter of the head and the IPL is < 1.2 cm, there is a 90 % probability of success of OPD. When that distance is > 3.3 cm, there is 90 % probability of cesarean section. When the percentage of head beyond the IPL was > 54 %, there was 90 % probability of successful OPD. DISCUSSION: Translabial ultrasound is useful in the prediction of the difficulty and the success of OPD. The higher the extent of head that passed the IPL, the less difficult the OPD and the greater the success rate of the OPD.

Fetal umbilical-portal-systemic venous shunt: in-utero classification and clinical significance.

OBJECTIVES: To review our experience with fetal umbilical-portal-systemic venous shunts (UPSVS), to devise an in-utero classification system, and to analyze their clinical and prognostic characteristics. METHODS: This was a retrospective review of cases of UPSVS examined at an academic tertiary referral center over the 14-year period from 2001 to 2014 inclusive. The anatomical origin and drainage of the fetal umbilical, portal, ductus venosus and hepatic venous systems, and the integrity of the intrahepatic portal venous system (IHPVS), were assessed using two- and three-dimensional color Doppler sonography. Fetomaternal clinical characteristics and long-term outcome were investigated by means of medical files, imaging documentation and telephone interviews with the mothers. RESULTS: Forty-four cases with UPSVS were reviewed. Prenatal classification was based on the anatomical origin of the shunt (umbilical, portal or ductal), with cases classified into four types: Type I, umbilical-systemic shunt (n = 9 (20.4%)); Type II, ductus venosus-systemic shunt (n = 19 (43.2%)); and Type III, portal-systemic shunt, divided into two subtypes: Type IIIa, intrahepatic portal-systemic shunt (n = 12 (27.2%)) and Type IIIb, extrahepatic portal-systemic shunt (n = 4 (9.1%)). Each type presented particular clinical and prognostic characteristics. The most important favorable prognostic factors were absence of associated major malformation and presence of the IHPVS. Thus, Type IIIa (intrahepatic portal-systemic shunt) had the best outcome. CONCLUSIONS: Fetal UPSVS are anomalies with a broad spectrum of manifestations and prognoses. Accurate mapping of the IHPVS is paramount for optimizing prenatal counseling and postnatal care. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Head progression distance in prolonged second stage of labor: relationship with mode of delivery and fetal head station.

OBJECTIVE: To evaluate the clinical significance of fetal head progression distance (HPD), measured by transperineal ultrasound, during prolonged second stage of labor. METHODS: In this prospective study, a single operator, who was blinded to the results of the digital examination, assessed using transperineal ultrasound women at ≥ 37 weeks of gestation with failure to progress in the second stage of labor. Patients had an empty urinary bladder and the examination was performed during maternal pushing. HPD was defined as the length of the line perpendicular to the infrapubic line that would connect it to the lowest part of the fetal bony skull. We analyzed associations between HPD and digital examination of fetal head station, fetomaternal characteristics, mode of delivery and perinatal outcome. RESULTS: Sixty-five patients in prolonged second stage of labor participated in the study. The overall mean HPD was 6.50 (± 1.35; 95% CI, 6.16-6.83) cm. No correlation was found between HPD and head position or mode of delivery, but HPD was positively correlated with fetal head station and neonatal head circumference measured after delivery. Logistic regression and receiver-operating characteristics curve analysis demonstrated no significant predictive value of HPD with respect to mode of delivery. CONCLUSION: Although HPD in prolonged second stage of labor could not predict mode of delivery, it may have a role as an ancillary tool for fetal head station assessment.

Pubic arch angle in prolonged second stage of labor: clinical significance.

OBJECTIVE: To evaluate the clinical significance of the pubic arch angle (PAA) measured by transperineal ultrasound during prolonged second stage of labor. METHODS: We evaluated prospectively 62 women ≥ 37 weeks of gestation with failure to progress in the second stage of labor. Transperineal ultrasound (transverse plane) was used to measure the pubic arch angle. Correlations with fetomaternal characteristics, mode of delivery and perinatal outcome were evaluated. RESULTS: The mean PAA was 101.1° (± 13.1°; range, 80°-135°). We found a negative correlation with maternal age. Patients with an occipitotransverse fetal position had a significantly smaller angle compared with those with occipitoanterior positions (94.3° ± 5.5° vs. 103.2° ± 14.8°, P < 0.05), as did those with operative deliveries compared with those with spontaneous vaginal delivery (97.1° ± 11.5° vs. 110.1° ± 14.0°, P < 0.05). The prediction of operative delivery in prolonged second stage of labor by receiver-operating characteristics curve using PAA alone yielded an area under the curve of 0.75. The predicted probability for operative delivery increased as PAA decreased, with an odds ratio of 0.933 for each decrease in angle of 1°. CONCLUSION: Our study suggests a correlation between the PAA and mode of delivery in prolonged second stage of labor. This may be used as an adjunctive parameter when considering delivery mode.

The fetal venous system, part I: normal embryology, anatomy, hemodynamics, ultrasound evaluation and Doppler investigation.

Since its introduction in the mid-1980s sonographic evaluation of the human fetal venous system has advanced dramatically. The venous system is well-recognized as a target for investigation in cases of circulatory compromise, and a broad spectrum of malformations affecting this system has been described. Appreciation of the normal embryology, anatomy and physiology of this system is essential to an understanding of structural anomalies and the sequential changes encountered in intrauterine growth restriction or other developmental disorders. We review the normal embryology, anatomy, and hemodynamics of the human fetal venous system, and provide an overview of Doppler investigation, as well as three- and four-dimensional ultrasound modalities and their application to this system.

The fetal venous system, Part II: ultrasound evaluation of the fetus with congenital venous system malformation or developing circulatory compromise.

The human fetal venous system is well-recognized as a target for investigation in cases of circulatory compromise, and a broad spectrum of malformations affecting this system has been described. In Part I of this review, we described the normal embryology, anatomy and physiology of this system, essential to the understanding of structural anomalies and the sequential changes encountered in intrauterine growth restriction and other developmental disorders. In Part II we review the etiology and sonographic appearance of malformations of the human fetal venous system, discuss the pathophysiology of the system and describe venous Doppler investigation in the fetus with circulatory compromise.

Fetal interlens distance: normal values during pregnancy.

OBJECTIVES: The aims of this study were to establish normal ranges of values for interlens distance (ILD) during gestation, and to assess the correlation between ILD and growth of the orbits and other fetal biometric parameters. METHODS: A cross-sectional study of morphologically normal, singleton fetuses was carried out in low-risk pregnant women. We measured the distance between the centers of the fetal lenses from 12 to 37 completed weeks of gestation. An oblique anterior coronal section was used to measure the ILD. The biorbital diameter (BOD) was measured in the same plane. We analyzed the correlation between the ILD and gestational age (GA), BOD, biparietal diameter (BPD), head circumference (HC), femur length (FL) and abdominal circumference (AC). Fetal gender was assessed to evaluate possible significant differences. RESULTS: Four hundred and fifty normal fetuses were recruited for the study. In 427 fetuses appropriate ILD measurements were obtained, and complete evaluation of all parameters was accomplished in 377 of these. ILD demonstrated a high positive correlation with GA (r(2) = 0.969), BPD (r(2) = 0.959), HC (r(2) = 0.962), AC (r(2) = 0.949) and FL (r(2) = 0.956) as pregnancy advanced. The mean BOD : ILD ratio was 1.50 +/- 0.08, remaining constant throughout pregnancy (linear regression r(2) = 0.006). No statistically significant differences were found between genders for mean ILD and GA of the study population (P = 0.604 and 0.595, respectively). CONCLUSIONS: The results of this study confirm previous hypotheses regarding the close relationship between eyeball growth and growth of the surrounding bony structures. Measurement of fetal ILD, which can be used interchangeably with BOD measurement in either the axial or coronal plane, can be taken to reflect the expected development of the fetal orbits whilst simultaneously providing valuable information on eyeball development. It is likely that ILD could be used as a complementary tool in the elaboration of various severe abnormalities, mainly related to craniofacial and nervous system malformations.

Prenatal diagnosis of congenital agenesis of the fetal portal venous system.

OBJECTIVE: To describe the prenatal diagnosis and review our experience of fetal congenital agenesis of the portal venous system (CAPVS) and to review the current literature on this poorly documented vascular malformation. METHODS: This was a retrospective survey covering the 12-year period between 1996 and 2008. The database of a single, large, ultrasonographic tertiary academic referral center in Israel was analyzed and cases with a prenatal diagnosis of CAPVS were identified. All fetuses underwent detailed biometric and structural ultrasound examinations and a precise anatomical description of the fetal umbilical, portal and hepatic venous system was noted, as well as the presence of aberrant vessels, shunt location and the presence or absence of the DV. Results of fetal echocardiography, karyotyping and toxoplasma, rubella, cytomegalovirus and herpes evaluations were determined. Medical records were evaluated. Diagnosis was confirmed by pathology, postmortem venography or neonatal ultrasound or venography. Liveborns were examined by a certified neonatologist and long-term follow-up from pediatric gastroenterology units was determined. RESULTS: Nine cases with CAPVS were studied. In all cases an aberrant umbilical-portal vein was the primary indication for detailed portal system evaluation. Five fetuses demonstrated total CAPVS (Type I) and four showed partial agenesis of the portal vein (Type II). Among the five Type I fetuses, there was a shunt from the umbilical vein to the inferior vena cava in three (60%), to the right atrium in one and to the coronary sinus in one. In this group, in only one case could we delineate a common confluence between the splenic vein and the superior mesenteric vein shunting to the inferior vena cava. In four cases termination of pregnancy was performed due to additional findings: one case with hydrothorax, ascites and mitral atresia, one with cleft lip/palate and one with trisomy 21. One case had no additional anomalies, but the parents elected to terminate the pregnancy. All four of the Type II fetuses had a portosystemic shunt: in two cases to the right atrium, in one to the iliac vein and in one to the right hepatic vein. In three, the shunt resolved spontaneously. In only one case was abnormal liver function present over a follow-up period of 2-10 years. CONCLUSION: CAPVS can be detected prenatally. An abnormal course of the umbilical vein necessitates prompt sonographic evaluation of the umbilical-portal venous system and meticulous investigation for additional anomalies. Complete CAPVS may be associated with remote clinical consequences of which the parents should be informed. Partial CAPVS has a favorable prognosis.

In-utero evaluation of the fetal umbilical-portal venous system: two- and three-dimensional ultrasonic study.

OBJECTIVES: To describe the normal anatomy of the fetal umbilical-portal venous system (UPVS) and to assess possible anatomical variants of the main portal vein (MPV) insertion into the portal sinus (PS). METHODS: This was a prospective cross-sectional study of low-risk patients between 14 and 36 weeks of gestation. Two- (2D) and three-dimensional (3D) ultrasound techniques combined with color and high-definition flow Doppler were used to evaluate the fetal UPVS. The standard transverse plane of the fetal upper abdomen, used for measuring the abdominal circumference, was taken in all cases as the point of reference. A longitudinal section was taken to identify the normal course of the umbilical vein and ductus venosus (DV). We performed offline analysis of all gray-scale and color Doppler 2D and 3D volume datasets. RESULTS: Two hundred and eight fetuses were included in the study. The umbilical vein was observed to course in a cephalad direction from its entry point into the fetal abdomen, joining the L-shaped PS, a confluence of vessels that is the main segment of the left portal vein (LPV). Three branches emerge from the LPV: two to the left, the inferior and superior branches, and one to the right, the medial branch. The main LPV then courses abruptly to the right. Following the emergence of the DV, the communication of the MPV with the LPV marks the point at which the vessel becomes the right portal vein (RPV), giving rise to its anterior and posterior branches. We were able to define three main variants of connection between the MPV and the PS. In 140 (67.3%) fetuses the MPV was connected to the LPV in an end-to-side T-shaped anastomosis, in 26 (12.5%) fetuses the MPV connected with a side-to-side X-shaped anastomosis and in 30 (14.4%) fetuses the two vessels ran in parallel with a short communicating segment, in an H-shaped anastomosis. In the remaining 12 (5.7%) cases classification into one of these three groups was not possible due to intermediate morphology. CONCLUSIONS: Knowing the normal anatomy of the UPVS and being aware of the possible variants of the connection between the MPV and the PS is a fundamental requirement for accurate prenatal diagnosis of the anomalies of the fetal UPVS.

Fetal aberrant right subclavian artery in normal and Down syndrome fetuses.

OBJECTIVES: To evaluate the prevalence of fetal aberrant right subclavian artery (ARSA) in a low-risk population and compare it with that among Down syndrome fetuses, in order to estimate its potential as a marker in Down syndrome screening. METHODS: Women undergoing routine fetal sonographic examination between 13 and 26 weeks of gestation were evaluated once for the presence of ARSA using either a transvaginal multifrequency 5-9-MHz probe or a transabdominal 4-8-MHz probe as appropriate. Early pregnancy outcome was obtained in all cases. RESULTS: Nine hundred and twenty-four fetuses were recruited. An ARSA was detected in 13 fetuses (1.4%) with normal karyotype. During the study period, eight fetuses with Down syndrome referred either with known karyotype or with signs suspicious for Down syndrome were evaluated by the same protocol. Three of eight fetuses (37.5%) were found to have an ARSA. In none of these cases was ARSA an isolated finding. The odds ratio for ARSA in Down syndrome compared with normal fetuses was 42.04 (95% CI, 9.08-194.6). CONCLUSIONS: An ARSA was found in 1.4% of the normal population. In the small group of Down syndrome fetuses we observed a trend towards a higher rate of ARSA than in normal fetuses. In none of the Down syndrome fetuses was ARSA an isolated finding. Larger prospective studies are needed to examine the significance of ARSA as an isolated finding and the potential of ARSA as a marker in Down syndrome screening.

Sonographic visualization of the middle phalanx of the fetal fifth digit between 13 and 17 weeks of gestation.

OBJECTIVE: To assess in utero visualization of the middle phalanx of the fifth digit (MPFD) between 13 and 17 weeks of gestation. METHODS: This was a prospective cross-sectional study in which women attending for fetal sonographic examination between 13 + 0 and 17 + 6 weeks of gestation were evaluated for the presence of the middle phalanx of the fifth digit (MPFD). A total of 682 fetuses with normal findings on first-trimester anatomical examination were recruited. Using transvaginal multifrequency 4-8 and 5-9 MHz probes, the fifth finger was examined in both axial and lateral views until optimal visualization of the MPFD was achieved. RESULTS: The MPFD was visualized in 14.3% (2/14) of cases at 13 weeks, 70.3% (154/219) at 14 weeks, 82.2% (240/292) at 15 weeks, 97.4% (111/114) at 16 weeks and 100% (43/43) at 17 weeks of gestation. CONCLUSIONS: Ultrasound visualization of the MPFD gradually increases during the 13- to 17-week period. This emphasizes the limited role of non-ossification of the MPFD as a sonographic marker of Down syndrome before 17 weeks of gestation.

Development of the human fetal pons: in utero ultrasonographic study.

OBJECTIVES: To examine the ultrasonographic feasibility of imaging the fetal pons and to construct a reference chart for its normal development during gestation. METHODS: A cross-sectional, prospective study on 293 healthy fetuses of low-risk pregnancies between 19 and 34 weeks was performed. The transfontanel approach, via the abdominal or vaginal routes, was used to evaluate the fetal metencephalon (pons and cerebellum). The anteroposterior diameter of the fetal pons was measured in a mid-sagittal plane. The longitudinal diameter of the cerebellar vermis was measured at the same plane and the vermis-pons ratio (VPR) was established. RESULTS: One hundred and forty-four fetuses were in vertex position. In 140 (97.2%) satisfactory visualization and measurements of the pons and cerebellar vermis were obtained. One hundred and forty-nine fetuses were breech presentations and measurements were successfully performed in 147 (98.6%). The pons anteroposterior and vermis longitudinal diameters showed a linear correlation with gestational age (GA) (r = 0.95 for both measurements; P < 0.001). The mean VPR was 1.5 (+/-0.1 SD) and did not change in the gestational interval that was considered. CONCLUSION: By using the transfontanel approach, evaluation of the fetal pons is feasible via the mid-sagittal plane. The nomograms developed and the ratio to fetal vermis provides reference data that may be helpful when evaluating anomalies of the brainstem.

Monozygotic twins following blastocyst transfer: a report of two cases.

Several studies have shown an increased risk for monozygotic twinning after fertilization treatments. We present the clinical and sonographic characteristics of two monozygotic twin cases following blastocyst transfer. It is suggested that delayed transfer of the embryo in the blastocyst stage is a contributing factor to monozygotic twinning.