Validation of Obstetric Diagnosis and Procedure Codes in the Danish National Patient Registry in 2017.

Purpose: This study aimed to systematically evaluate the validity of variables related to pregnancy, delivery, and key characteristics of the infant in the Danish National Patient Register using maternal medical records as the reference standard. Patients and Methods: We reviewed medical records of 1264 women giving birth in the Region of Southern Denmark during 2017. We calculated positive (PPV) and negative (NPV) predictive values, sensitivity, and specificity to estimate the validity of 49 selected variables. Results: The PPV was ≥0.90 on most pregnancy-related variables including parity, pre-gestational BMI, diabetes disorders, and previous cesarean section, while it was lower for hypertensive disorders, especially mild to moderate preeclampsia (0.49, 95% CI 0.32-0.66). Sensitivity ranged from 0.80 to 1.00 on all pregnancy-related variables, except hypertensive disorders (sensitivity 0.38-0.71, lowest for severe preeclampsia). On most delivery-related variables including obstetric surgical procedures (eg cesarean section and induction of labor), pharmacological pain-relief, and gestational age at delivery, PPV's ranged from 0.98 to 1.00 and the corresponding sensitivities from 0.87 to 1.00. Regarding infant-related variables, both the APGAR score registered five minutes after delivery and birthweight yielded a PPV of 1.00. Conclusion: Obstetric coding in the Danish National Patient Register shows very high validity and completeness making it a valuable source for epidemiologic research.

Use of opioids among pregnant women 1997-2016: A Danish drug utilization study.

OBJECTIVE: Use of opioids in pregnancy is of concern yet little is known on opioid prescription patterns in Denmark. The aim of this drug utilization study was to describe prescription patterns for opioids during pregnancy in Denmark from 1997 to 2016. STUDY DESIGN: Using the nationwide health care registers, we obtained information on all women with a registered pregnancy in the period 1 January 1997 to 31 December 2016. Opioids were grouped in four: opioids (N02A except codeines), opioid dependency medications (N07BC), cough medications (R05DA except codeines), and codeines (N02AJ06, N02AJ07, N02BA75, and R05DA04). We used logistic regression analyses to identify factors associated with opioid use in pregnancy and cumulative oral morphine equivalent (OMEQ) to estimate volume of use in pregnancy. RESULTS: Prescription patterns were similar for women with live births, non-live births, and terminations. Total use of opioids among women with live born deliveries remained stable at 19.8 per 1000 pregnancies from 1997 to 2016. Codeine use declined from 2008 onwards, while use of other opioids increased from 2007 onwards. This was dominated by a threefold increase in tramadol use (2.0-7.6 per 1000 pregnancies with live births). Codeine was the most used opioid, followed by tramadol and codeine combined with paracetamol. The number of women, who used opioids before pregnancy and continued into their pregnancy, was reduced as the pregnancy progressed. The cumulative oral morphine equivalent during pregnancy was stable until 2007, after which, use prior to pregnancy and during the first two trimesters increased. The odds ratios for opioid use were higher in pregnancies of women of lower socioeconomic status or older age. For live births, odds ratios for opioid use in pregnancy were higher among women with obesity or smoking. CONCLUSIONS: Overall use of opioids was stable from 2007 to 2016. This covers a decline in the use of codeine, but a 3-fold increase in tramadol. The number of pregnant women who continued use throughout pregnancy decreased, while OMEQ among persistent users increased. The real-world data suggest an unmet need of specific focus in local Danish Outpatient Clinics and Multidisciplinary Pain Centers both pre-conceptionally and during pregnancy.

Real-World Experience With CPX-351 Treatment for Acute Myeloid Leukemia in England: An Analysis From the National Cancer Registration and Analysis Service.

BACKGROUND: CPX-351 demonstrated improved overall survival (OS) versus conventional 3 + 7 daunorubicin/cytarabine chemotherapy in a registrational phase III study in older patients with newly diagnosed, high-risk secondary acute myeloid leukemia (AML). This retrospective, population-based cohort study aimed to describe and compare the characteristics and survival outcomes of younger (<60 years) versus older (≥60 years) patients with AML treated with CPX-351 in England. PATIENTS AND METHODS: The study included adults aged ≥18 years diagnosed with AML in England between January 2013 and March 2022, and treated with CPX-351 in routine clinical practice (patients who received CPX-351 in a clinical trial were excluded). Patient records were sourced from the population-level cancer analysis system database available through the National Cancer Registration and Analysis Service. RESULTS: Of 353 included patients, 104 (29.5%) were <60 years. With a median follow-up of 10.9 months from diagnosis, the estimated median OS was 12.9 months overall, 17.3 months for adults <60 years and 11.7 months for those ≥60 years. All-cause mortality by Day 30 from diagnosis was 6% overall, 4% for adults <60 years and 6% for those ≥60 years. Hematopoietic cell transplantation (HCT) was received by 54% of adults <60 years and 38% of those ≥60 years after CPX-351, with median OS landmarked from the HCT date not yet reached for either age subgroup. CONCLUSION: This study provides real-world survival outcomes data suggesting that CPX-351 is an effective treatment for both younger (<60 years) and older (≥60 years) adults with AML.

Development and Validation of a Nordic Multimorbidity Index Based on Hospital Diagnoses and Filled Prescriptions.

Purpose: To develop the Nordic Multimorbidity Index (NMI), a multimorbidity measure specifically suited to the Nordic health and administrative registry data based on current diagnosis, treatment, and coding practices. Methods: The NMI was developed to predict 5-year mortality in a population-based cohort of randomly sampled Danish residents aged ≥40 years (n = 425,087) followed from 2013 to 2018. Included predictors were selected from hospital diagnoses and filled drug prescriptions based on a combination of subject matter knowledge and a data-driven approach using backwards elimination. The performance of the NMI was assessed in a temporal validation cohort of Danish residents followed from 2007 to 2012 and in six cohorts of new users of selected drugs. The discriminative performance of the NMI, Charlson Comorbidity Index (CCI) and the Elixhauser Comorbidity Index (ECI) was assessed using the c-statistic from logistic regression models with 5-year mortality as dependent variable and the multimorbidity index score, age, and sex as independent variables. Results: The NMI included 50 predictors. In the temporal validation cohort, the c-statistic of the NMI (0.887, 95% CI 0.883-0.890) exceeded that of the CCI (0.871, 95% CI 0.868-0.874) and ECI (0.866, 95% CI 0.863-0.870). In all new user cohorts, the NMI outperformed the other indices with c-statistics ranging from 0.781 (95% CI 0.779-0.784) to 0.838 (95% CI 0.834-0.842). Conclusion: The NMI predicted 5-year mortality in a general Danish population and six cohorts of new users of selected drugs and was superior to the CCI and ECI. The NMI could be preferred over these indices to quantify the level of multimorbidity for, eg, descriptive purposes or confounding control. The NMI should be validated in other patient populations and other Nordic countries.

Prenatal exposure to nitrofurantoin and risk of childhood leukaemia: a registry-based cohort study in four Nordic countries.

BACKGROUND: Studies have suggested increased risks of childhood leukaemia after prenatal exposure to antibiotics, particularly nitrofurantoin. However, these findings may be related to the underlying maternal infection. This multinational study aimed to investigate the association between prenatal nitrofurantoin exposure and childhood leukaemia while accounting for maternal infection. METHODS: In a population-based cohort study of children born in Denmark, Finland, Norway or Sweden from 1997 to 2013, prenatal exposure to nitrofurantoin or pivmecillinam (active comparator) was ascertained from national Prescription Registries. Childhood leukaemia was identified by linkage to national Cancer Registries. Poisson regression was used to estimate incidence rate ratios (IRRs) and incidence rate differences (IRDs) with inverse probability of treatment weights applied to account for confounding. RESULTS: We included 44 091 children prenatally exposed to nitrofurantoin and 247 306 children prenatally exposed to pivmecillinam. The children were followed for 9.3 years on average (standard deviation 4.1). There were 161 cases of childhood leukaemia. The weighted IRR for prenatal nitrofurantoin exposure when compared with pivmecillinam was 1.34 (95% confidence interval 0.88, 2.06), corresponding to an IRD of 15 per million person-years. Higher point estimates were seen for first- and third-trimester exposure. There was no evidence of a dose-response relationship. CONCLUSIONS: Prenatal exposure to nitrofurantoin was not substantially associated with childhood leukaemia, although a slightly elevated IRR with confidence intervals including the null was observed, corresponding to a small absolute risk. The lack of a dose-response relationship and a clear biological mechanism to explain the findings suggests against a causal association.

Use of sildenafil and other phosphodiesterase type 5 inhibitors among pregnant women in Scandinavia.

INTRODUCTION: For phosphodiesterase type 5 (PDE5) inhibitors, such as sildenafil, the only approved indication in women is for pulmonary arterial hypertension. These drugs are increasingly being proposed and tested for treatment of female infertility and complications in pregnancy. However, the extent of use of PDE5 inhibitors in the general pregnant population over the last decades is unknown. Therefore, we conducted a descriptive cohort study using data from the population health registers in the Scandinavian countries. MATERIAL AND METHODS: By linking the Medical Birth Registers and the Prescribed Drug Registers in Denmark (1997-2017), Norway (2004-2017), and Sweden (2006-2016), women with filled prescriptions of PDE5 inhibitors in outpatient settings in the 90 days before the date of last menstrual period and/or during pregnancies were identified. With additional linkage to the National Patient Registers, information on maternal, pregnancy, and infant characteristics, co-morbidities, and co-medication was collected and described. RESULTS: Among over 3 million singleton pregnancies, only 77 were pregnancies in women who had at least one filled prescription of a PDE5 inhibitor within the 90 days before the start of pregnancy to delivery. Prescription fills most often occurred before the last menstrual period and in the first trimester, with very few occurring later in pregnancy. Sildenafil was the most used PDE5 inhibitor. Among pregnant women using PDE5 inhibitors, 44% were 35 years of age or older, eight had a cardiovascular diagnosis, and three specifically had a diagnosis of pulmonary arterial hypertension. Among the infants born to mothers using PDE5 inhibitors, nine were born preterm, six were small-for-gestational age, five had an Apgar score at 5 minutes below 8, 18 were admitted to the Neonatal Intensive Care Unit, and eight had respiratory and cardiovascular conditions. CONCLUSIONS: Few women used PDE5 inhibitors in outpatient settings before or during pregnancy in the Scandinavian countries in the last decades. Only a small proportion had a diagnosis for pulmonary arterial hypertension, suggesting off-label use in the remaining users. Use was predominantly in mothers over age 35 years. The safety of fetal exposure to sildenafil and other PDE5 inhibitors in pregnancy has not been established. As maternal age continues to increase and additional uses of PDE5 inhibitors are investigated, the safety of these drugs in pregnancy should be thoroughly evaluated.

Maternal Medication Use and Childhood Cancer in Offspring-Systematic Review and Considerations for Researchers.

Cancer is an important cause of childhood mortality, yet the etiology is largely unknown. A combination of pre- and postnatal factors is thought to be implicated, including maternal medication use. We aimed to provide: 1) a systematic review of peer-reviewed publications on associations between maternal medication use and childhood cancer, with a focus on study design and methodology; and 2) suggestions for how to increase transparency, limit potential biases, and improve comparability in studies on maternal medication use and childhood cancer. We conducted a systematic search in the PubMed, Embase, Scopus, Cochrane, and Web of Science databases to June 8, 2020. Altogether, 112 studies were identified. The reviewed studies were heterogeneous in study design, exposure, and outcome classification. In 21 studies (19%), the outcome was any childhood cancer. Of the 91 papers that reported on specific types of cancer, 62% did not report the cancer classification system. The most frequently investigated medication groups were sex hormones (46 studies, excluding fertility medications), and antiinfectives (37 studies). Suggestions for strengthening future pharmacoepidemiologic studies on maternal medication use and childhood cancer relate to choice of cancer classification system, exposure windows, and methods for identification of, and control for, potential confounders.

Congenital Malformations in Denmark: Considerations for the Use of Danish Health Care Registries.

BACKGROUND: Danish health registers are used widely to examine associations between specific risk factors and congenital malformations. Various overall prevalence rates of malformations have been reported in Denmark indicating differences in the underlying data sources or malformation definitions. We described trends in registration of malformations in Denmark 1997-2017 and identified potential caveats for the use of Danish health registries in epidemiological studies. We composed a Danish adaptation of EUROCATs definition of malformations. METHODS: Using nationwide Danish health registries, we identified all recorded pregnancies and followed livebirths for up to 5 years. We described the different data sources, ways to identify malformations, the overall rate of malformations over time, and identified the 10 most common major malformations. RESULTS: A total of 1,340,774 foetuses and infants from 1,313,281 pregnancies among 747,144 women from 1997 to 2017 were analysed. Using primary and secondary diagnoses from all available sources and restricting hip malformations to diagnoses after 6 weeks postpartum, we found that 65,411 (49/1000) foetuses or infants had at least one major malformation defined by our Danish translation of EUROCATs definition of malformations. The prevalence of major malformations increased over time from 39/1000 in 1997 to 53/1000 in 2017. The most common specific malformations were malformations of cardiac septa (Q21) and great arteries (Q25) with a peak of 10 and 6/1000 births in 2010 and 2009, respectively. CONCLUSION: Malformations should be identified using primary and secondary diagnoses from the Birth register, the Patient register, and the Cause of Death register. To increase transparency and external validity, classification of major malformations should be based on the Danish adaptation of EUROCATs classification of malformations.

Ischemic Stroke and Systemic Embolism in Warfarin Users With Atrial Fibrillation or Heart Valve Replacement Exposed to Dicloxacillin or Flucloxacillin.

The antibiotics dicloxacillin and flucloxacillin induce cytochrome P450-dependent metabolism of warfarin. We explored the influence of these drug-drug interactions on the clinical effectiveness of warfarin therapy due to atrial fibrillation or heart valve replacement. Using the population-based Danish registers, we performed a propensity-score matched cohort study including around 50,000 episodes of dicloxacillin/flucloxacillin matched to phenoxymethylpenicillin and to no antibiotic, respectively. We estimated hazard ratios (HRs) with 95% confidence intervals (CIs) by comparing 21-day (days 7-28) risks of ischemic stroke/systemic embolism (SE) following initiation of each exposure. When compared with phenoxymethylpenicillin, dicloxacillin/flucloxacillin was associated with an HR of ischemic stroke/SE of 2.09 (95% CI 1.51-2.90; strongest for dicloxacillin (HR 2.17; 95% CI 1.56-3.02)). Use of an untreated comparator strengthened the association (HR 2.84; 95% CI 1.97-4.09). Dicloxacillin should be used with caution in patients receiving warfarin. This may also apply to flucloxacillin; however, more data on the risks associated with flucloxacillin exposure during warfarin therapy are needed.

Phthalate exposure from drugs during pregnancy and possible risk of preterm birth and small for gestational age.

BACKGROUND: Phthalates are chemical compounds present in a wide range of consumer products and are thought to be endocrine disruptors. Though not commonly known, phthalates are present in some medication with previous studies finding up to 50-fold higher urinary metabolite concentrations among exposed compared to the general population. Previous studies on environmental phthalate exposure and pregnancy outcomes have been contradictory and inconclusive and all previous studies have assessed phthalate exposure using biomarkers despite a known rapid metabolism of phthalates. OBJECTIVE: To determine whether phthalate exposure from pharmaceutical drugs have effects on preterm birth (PTB) and small for gestational age (SGA). STUDY DESIGN: We conducted a nested case-control study among women in Denmark with a recorded singleton birth and included women who conceived between January 1st, 2004 and December 31st, 2015. To mitigate drug effect and confounding by underlying disease we included pregnancies exposed to selected study drugs, and compared pregnancies exposed to phthalate containing drugs to pregnancies exposed to phthalate free generic drugs. Using Danish health registries, we identified 30,899 singleton pregnancies exposed to study drugs available in both phthalate-containing and phthalate free versions. Using conditional logistic regression, we estimated associations between phthalate exposure and the risk of PTB and SGA. Birth weight according to gestational age was defined by INTERGROWTH-21st (SGA-I) and by Marsal's equation (SGA-M) for expected birthweight. RESULTS: We included 1965 PTBs, 1315 SGA-Is, and 891 SGA-M cases, matched to 19,537, 12,008, and 7573 controls, respectively. Orthophthalate exposure during the third trimester was positively associated with PTB with a crude OR of 1.36 (95% CI: 1.06-1.76). The association was mainly due to diethyl phthalate. Exposure to phthalate polymers in third trimester was associated with a risk of PTB with crude ORs of 2.08 (CI: 1.16-3.71. No associations were found between orthophthalate or phthalate polymer exposure and SGA. CONCLUSION: Exposure to some phthalate-containing pharmaceutical drugs during third trimester is associated with preterm birth.

In utero exposure to antibiotics and risk of congenital malformations: a population-based study.

BACKGROUND: Antibiotics are commonly prescribed during pregnancy. Although the safety of most penicillins is well established, some controversy and uncertainty are associated with the use of other commonly prescribed antibiotics. OBJECTIVE: To determine the risk of congenital malformations following first-trimester in utero exposure to 10 commonly prescribed antibiotics in Denmark. MATERIALS AND METHODS: This was a cohort study comprising all singleton liveborn children in Denmark between 2000 and 2015. Data on malformations were collected through 2016. Merging validated and comprehensive populationwide Danish healthcare and civic registries, we merged data on pregnancy, prescription drugs purchases during first trimester and congenital malformations. Using logistic regression, we calculated the odds ratio for congenital malformations (any), major congenital malformations, and cardiac congenital malformations for the 10 most commonly prescribed antibiotics (excluding 4 penicillins that served as control). In the primary analysis, the exposed cohort was compared to a cohort exposed to any of 4 penicillins considered safe during pregnancy (ampicillin, pivampicillin, benzylpenicillin, and phenoxymethylpenicillin). In sensitivity analysis, the exposed cohort was compared to an unexposed cohort. Covariate adjustments were made for maternal age at delivery, year of delivery, parity, pre-pregnancy body mass index, smoking, educational status, employment status, and annual personal income. RESULTS: We found no increased risk of congenital malformations to be related to first-trimester in utero exposure to the 10 most commonly prescribed antibiotics in Denmark compared to a cohort of pregnant women exposed to penicillins that are considered safe during pregnancy. Compared to unexposed pregnancies, small increased risks for major malformations and cardiac malformations were apparent for pivmecillinam (odds ratio, 1.13; confidence interval, 1.06-1.19; and odds ratio, 1.15; confidence interval, 1.04-1.28, respectively), sulfamethizole (odds ratio, 1.15; confidence interval, 1.07-1.24; and odds ratio, 1.22; confidence interval, 1.07-1.39, respectively), and azithromycin (odds ratio, 1.19, confidence interval, 1.03-1.38; and odds ratio, 1.29, confidence interval, 0.99-1.67, respectively). CONCLUSION: In this large populationwide cohort study, we found, with a high degree of precision, no increased risk of congenital malformations following first-trimester exposure to 10 commonly prescribed systemic antibiotics.

Phthalate Exposure and Breast Cancer Incidence: A Danish Nationwide Cohort Study.

PURPOSE: Phthalate exposure is ubiquitous and especially high among users of drug products formulated with phthalates. Some phthalates mimic estradiol and may promote breast cancer. Existing epidemiologic studies on this topic are small, mostly not prospective, and have given inconsistent results. We estimated associations between longitudinal phthalate exposures and breast cancer risk in a Danish nationwide cohort, using redeemed prescriptions for phthalate-containing drug products to measure exposure. METHODS: We ascertained the phthalate content of drugs marketed in Denmark using an internal Danish Medicines Agency ingredient database. We enrolled a Danish nationwide cohort of 1.12 million women at risk for a first cancer diagnosis on January 1, 2005. By combining drug ingredient data with the Danish National Prescription registry, we characterized annual, cumulative phthalate exposure through redeemed prescriptions. We then fit multivariable Cox regression models to estimate associations between phthalate exposures and incident invasive breast carcinoma according to tumor estrogen receptor status. RESULTS: Over 9.99 million woman-years of follow-up, most phthalate exposures were not associated with breast cancer incidence. High-level dibutyl phthalate exposure (≥ 10,000 cumulative mg) was associated with an approximately two-fold increase in the rate of estrogen receptor-positive breast cancer (hazard ratio, 1.9; 95% CI, 1.1 to 3.5), consistent with in vitro evidence for an estrogenic effect of this compound. Lower levels of dibutyl phthalate exposure were not associated with breast cancer incidence. CONCLUSION: Our results suggest that women should avoid high-level exposure to dibutyl phthalate, such as through long-term treatment with pharmaceuticals formulated with dibutyl phthalate.

Patterns and predictors for prescription of psychotropics and mood-stabilizing antiepileptics during pregnancy in Denmark 2000-2016.

AIMS: To analyse prescribing patterns during pregnancy for antipsychotics (APs), antidepressants (ADs) and mood-stabilizing antiepileptics (AEDs) in Denmark from 2000 to 2016. METHODS: Data were obtained from the Danish Medical Birth Register, the Register for Legally Induced Abortions, the Danish National Patient Register and the Register of Medicinal Product Statistics. Data were linked through a unique personal identifier by Statistics Denmark. RESULTS: The use of APs increased 2.5-fold from a prevalence of 1.5 per 1000 pregnancies to 3.8 for pregnancies ending in a delivery. Use of mood-stabilizing AEDs increased from a prevalence of 0.1 to 2.1 during the study period. The prevalence for APs and mood-stabilizing AEDs was nearly twice as high for pregnancies ending in miscarriage or termination compared to pregnancies ending in delivery. A marked increase in the prevalence of ADs use during pregnancy was seen from 2000-2011 (from 6 to 41 per 1000 pregnancies ending in a delivery) but appears slightly in decline. Age, smoking, obesity and social status were generally associated with increased use of psychotropic drugs. CONCLUSIONS: The use of APs, ADs and mood-stabilizing AEDs during pregnancy has increased substantially in Denmark from 2000-2016. The use of ADs appears to be slightly in decline since 2011.

Cumulative exposure to phthalates from phthalate-containing drug products: a Danish population-wide study.

AIMS: Up to 50-fold higher levels of urinary phthalate metabolites have been observed in users of phthalate-containing drug products compared with non-users. This is of concern, as phthalates are suspected endocrine disrupters and have been associated with cancer development. This study aims to quantify annual cumulated phthalate exposure from drug products among users of phthalate-containing oral medications in Denmark throughout the period of 2004-2016. METHODS: We conducted a Danish nationwide cohort study using The Danish National Prescription Registry and an internal database held by The Danish Medicines Agency. These databases hold information on drug products; date of dispensing, and the type and quantity of excipients in drugs with Danish marketing permission. We present the number of users over time and their distribution of exposure to enteric phthalate polymers and ortho-phthalates. RESULTS: The annual number of individuals exposed to phthalate-containing products declined during 2004-2016. The total number of individuals exposed to dibutyl phthalate declined from 21 499 in 2004 to 5400 in 2016. However, among those exposed, the median dibutyl phthalate exposure remained above European regulatory limit of exposure ranging between 380-1710 mg/year throughout the study period. Lithium-products constituted the majority of dibutyl phthalate exposure. Diethyl phthalate exposure, mainly caused by erythromycin, theophylline and diclofenac products, did not exceed the EMA regulatory limit. CONCLUSION: While the number of individuals exposed to phthalates from oral medications during 2004-2016 declined, the use of phthalate-containing drugs is still considerable.

Use of Dicloxacillin and Risk of Pregnancy among Users of Oral Contraceptives.

The antibiotic dicloxacillin has been shown to induce drug-metabolizing CYP enzymes to a clinically relevant extent. In this study, we investigated whether the use of dicloxacillin confers an increased risk of unwanted pregnancy among oral contraceptive users. The study population comprised Danish women falling pregnant (1997-2015) during oral contraceptive use, defined as having filled a prescription for an oral contraceptive within 120 days both before and after the estimated date of conception. Data were analysed using a case-crossover approach. For each woman, we assessed the use of dicloxacillin preceding the date of conception and during 10 previous control periods and estimated the odds ratio for such unintended pregnancies associated with the use of dicloxacillin. Among 364 women using dicloxacillin prior to conception, 40 (11%) were exposed to dicloxacillin at the time of conception, yielding an odds ratio (OR) associating use of dicloxacillin to unintended pregnancy of 1.18 (95% CI 0.84-1.65). Supplementary and sensitivity analyses generally returned similar estimates, except for a slightly increased risk among users of progestogen-only oral contraceptives (OR 1.83, 95% CI 0.63-5.34). Analysis of other antibiotics as negative controls yielded results close to unity (ORs ranging from 0.83 to 1.13). In conclusion, our study found no evidence for an increased risk of oral contraceptive failure when using dicloxacillin. However, acknowledging study limitations, we suggest the use of supplementary barrier methods during treatment with dicloxacillin, until our findings are confirmed in further studies.

The Danish Medical Birth Register.

The Danish Medical Birth Register was established in 1973. It is a key component of the Danish health information system. The register enables monitoring of the health of pregnant women and their offspring, it provides data for quality assessment of the perinatal care in Denmark, and it is used extensively for research. The register underwent major changes in construction and content in 1997, and new variables have been added during the last 20 years. The aim was to provide an updated description of the register focusing on structure, content, and coverage since 1997. The register includes data on all births in Denmark and comprises primarily of data from the Danish National Patient Registry supplemented with forms on home deliveries and stillbirths. It contains information on maternal age provided by the Civil Registration System. Information on pre-pregnancy body mass index and smoking in first trimester is collected in early pregnancy (first antenatal visit). The individual-level data can be linked to other Danish health registers such as the National Patient Registry and the Danish National Prescription Registry. The register informs several other registers/databases such as the Danish Twin Registry and the Danish Fetal Medicine Database. Aggregated data can be publicly accessed on the Danish Health Data Authority web page ( www.esundhed.dk/sundhedsregistre/MFR ). Researchers can obtain access to individual-level pseudo-anonymised data via servers at Statistics Denmark and the Danish Health Data Authority.