Pregnancies complicated by pre-existing diabetes are at high risk of adverse outcome, including stillbirth, perinatal mortality, congenital anomaly, Caesarean section and macrosomia [1, 2]. The global prevalence of type 2 diabetes is increasing particularly at younger ages, resulting in an increasing proportion of pregnancies complicated by diabetes. Congenital anomalies are a major cause of stillbirth and neonatal death for babies born to women with diabetes [2, 3] and a substantial proportion end in termination of pregnancy. They are also important contributors to mortality and morbidity throughout infancy and childhood, and survivors may have considerable ongoing health and social care needs.

The risk of congenital anomaly in women with diabetes is strongly associated with glycaemic control, indicated by higher levels of HbA1c in pregnancies affected by congenital anomaly [46]. However, similar rates of congenital anomaly have been reported in women with type 1 and 2 diabetes, despite generally lower HbA1c levels in type 2 diabetes [1]. This may reflect differences in other variables that are associated with congenital anomaly risk, such as maternal age, BMI, smoking, ethnicity and socioeconomic status. Previous studies have not assessed the extent to which these factors may modify the effect of glycaemia in the development of congenital anomaly in women with diabetes.

This study combined data from established population-based registers with comprehensive ascertainment to quantify the risk of major congenital anomaly in pregnancy in women with type 1 and type 2 diabetes, and to assess the influence of clinical and sociodemographic risk factors in addition to peri-conception HbA1c.


Study population

The study area in the north of England (UK) has a population of about 3 million and 31,000 deliveries per year. This analysis included all singleton pregnancies to women resident in the region, resulting in live birth, stillbirth (≥24 weeks gestation), late fetal loss (20–23 weeks gestation), or termination of pregnancy following prenatal diagnosis of a fetal anomaly (any gestation), during the period 1996–2008.

Pregnancies in women with and without pre-existing diabetes

The Northern Diabetes in Pregnancy Survey (NorDIP) records details of all known pregnancies, irrespective of outcome, in women resident in the study area and diagnosed with diabetes at least 6 months prior to conception [7]. Pregnancies in women with gestational diabetes (i.e. hyperglycaemia first diagnosed during pregnancy) are not included. Demographic and clinical variables are collected, including pre-pregnancy and antenatal HbA1c (DCCT-aligned since 2000). The total number of registered singleton live and stillbirths was obtained from the UK Office for National Statistics.

Congenital anomaly cases

The Northern Congenital Abnormality Survey (NorCAS) collects information on all cases of congenital anomaly (up to six anomalies for each case) diagnosed to age 12 years, including those arising in fetal loss or termination of pregnancy for fetal anomaly. The register uses multiple sources of ascertainment [8]. The NorDIP and NorCAS are held on a single linked database at the Regional Maternity Survey Office in Newcastle.

Classification of congenital anomalies

All major congenital anomalies were coded according to the International Classification of Diseases 10th revision (ICD-10; and categorised using European surveillance of congenital anomalies (EUROCAT) criteria (, by group (the system affected), subtype (the individual disorder), and syndrome (where applicable). Chromosomal anomalies were defined as any anomaly in the number of chromosomes or in the structure of at least one chromosome resulting in a genetically unbalanced genotype (ICD-10 codes: Q90–92, Q93, Q96–99). Non-chromosomal anomalies are all remaining major congenital anomalies included in the EUROCAT classification scheme [9, 10].

Isolated cases (with one anomaly diagnosis only) were assigned to their primary anomaly group and subtype. Cases with two or more non-chromosomal anomalies were reviewed to identify a primary group or subtype, or to confirm a diagnosis of multiple anomalies. Cases were classified as multiple anomalies if they had two or more unrelated anomalies across separate organ systems. Individuals with several anomalies from the same organ system were included within that group but not classified by subtype. A congenital anomaly was classified as isolated if it occurred alone, or if all coexisting anomalies were commonly associated secondary anomalies. Chromosomal anomalies, syndromes (patterns of anomalies arising from a single cause, e.g. genetic disorders [11]), skeletal dysplasias (syndromes of skeletal development [10]), sequences (patterns of anomalies arising from a prior anomaly or mechanical factor [12]), associations (recognised patterns of anomalies of unknown cause [11]) and other microdeletions, were regarded as primary anomalies rather than instances of multiple anomalies.

Statistical analyses

Prevalence rates of congenital anomaly, by group and subtype, were determined for women with and without diabetes and compared by calculating the RR, and 95% CIs for prevalence rates were calculated using exact methods. Numbers of cases are presented only for groups and subtypes where there was at least one case in pregnancies with diabetes. Rates and RRs (95% CI) for the subtypes of congenital anomalies are presented if there were three or more cases in pregnancies with diabetes. Heterogeneity of RRs between anomaly groups was examined using Cochran's Q test.

ORs and associated 95% CIs for non-chromosomal congenital anomalies among women with diabetes were estimated for various sociodemographic and clinical variables using logistic regression. Independent effects were estimated from an adjusted model, constructed using backwards stepwise regression. All variables with an unadjusted p value below 0.5 were entered into the model (maternal age at delivery, gestational age at booking, peri-conception HbA1c, type of diabetes, preconception folic acid, nephropathy diagnosed pre-pregnancy, retinopathy diagnosed pre-pregnancy, fetal sex, parity, pre-pregnancy care, index of multiple deprivation, smoking during pregnancy). Variables were then iteratively removed until all remaining had p < 0.1. The multivariate analysis had at least adequate power (β = 0.8) to detect a medium effect (Cohen’s d = 0.5, equivalent to OR of 2.47) for any variable with a baseline exposure probability between 5% and 95% (which included type 2 diabetes, non-white ethnicity, preconception folate consumption, pre-pregnancy care, smoking during pregnancy). Greater power was available for the continuous variables (duration of diabetes, maternal age at delivery, maternal BMI at booking, gestational age at booking, and peri-conception HbA1c).

Interaction terms were used to examine whether variables in the adjusted model had the same effect on the risk of congenital anomalies in women with type 2 compared with type 1 diabetes. The relative contributions of variables in the adjusted model were approximated by estimating the standardised β coefficients, which allow the importance of continuous and non-continuous variables to be directly compared [13]. HbA1c was analysed as a single peri-conception variable, using measurement closest to conception (within three months of conception) where available (48.4% of pregnancies) and mean first trimester value (up to 14 weeks gestation) otherwise. BMI, determined from height and weight at booking, was included as a continuous variable, excluding underweight women due to potential curvilinearity [14]. The index of multiple deprivation (IMD), an area-based measure of socioeconomic status, was determined from maternal residential postcode at booking and grouped into tertiles [15]. Locally weighted scatter plot smoothing (LOWESS), with smoothing parameter 0.8, was used to investigate the shape of the relationship between HbA1c, as a continuous variable, and the risk of congenital anomaly. CIs for the LOWESS plot were estimated by bootstrapping (50,000 iterations).

Statistical analyses were performed using SPSS for Windows version 17.0 (IBM Corporation, Somers, NY, USA) and Stata 11.1 (StataCorp, College Station, TX, USA). P < 0.05 was considered statistically significant.

Ethics approval and research governance

NorCAS, as part of the British Isles Network of Congenital Anomaly Registers, has exemption from the UK National Information and Governance Board (PIAG 2-08(e)/2002 20/06/2002) from a requirement for individual consent and has ethics approval (09/H0405/48) to undertake studies using the data. Newcastle Research Ethics Committee originally granted approval for the NorDIP in 1993, and data are now obtained and held with informed consent.


Study population

Overall, 401,149 singleton live births, stillbirths, late fetal losses, and terminations of pregnancy were recorded during the study period, including 1,677 in women with pre-existing diabetes, giving a prevalence of 4.2 per 1,000 (95% CI 4.0, 4.4) pregnancies.

Among women with diabetes, median (interquartile range, IQR) maternal age at delivery was 30 (25–24) years; 649 (40.1%) women were primiparous and the median (IQR) peri-conception HbA1c was 7.9% (6.8–9.2). A total of 1314 (78.4%) women had type 1 and 363 (21.6%) had type 2 diabetes. There were significant differences in the characteristics of women with type 1 and type 2 diabetes (Tables 1 and 2). Overall reported preconception folate consumption was low, but not significantly different in women with type 1 and type 2 diabetes (p = 0.06).

Table 1 Characteristics of mothers with type 1 and type 2 diabetes (continuous variables)a
Table 2 Characteristics of mothers with type 1 and type 2 diabetes (categorical variables)a

Risk of congenital anomaly

A total of 9,488 singleton pregnancies were affected by at least one major congenital anomaly, including 129 in women with diabetes. The risk of a pregnancy affected by any major congenital anomaly in women with diabetes was over three times higher than the background population (RR 3.3 [95% CI 2.8, 3.9]; Table 3). There was no difference in the proportion of affected pregnancies ending in termination for fetal anomaly in women with and without diabetes: 23 (18%) vs 1,811 (19%); RR 0.9 (95% CI 0.6, 1.3).

Table 3 Rates (95% CI) of major groups and selected subtypes of congenital anomaliesa in pregnancies of women with and without pre-existing diabetes per 1000 singleton pregnancies and RR (95% CI%)

The prevalence of major congenital anomaly per 1,000 pregnancies was 82.2 (95% CI 67.9, 98.3) in women with type 1 diabetes and 57.9 (95% CI 36.2, 87.1) in women with type 2. There was no significant difference in risk of congenital anomaly by type of diabetes (RR 1.4 [95% CI 0.9, 2.2] for type 1 vs type 2).

There was no evidence of increased risk of chromosomal anomalies in women with diabetes (RR 1.2 [95% CI 0.6, 2.4]). Excluding chromosomal anomalies, the relative risk of affected pregnancy for women with diabetes was 3.8 (95% CI 3.2, 4.5). There was significant variation in relative risk between different groups of non-chromosomal anomaly (p = 0.05), attributable to a 12-fold increase for the sequence group (including caudal dysplasia sequence, sirenomelia and partial urorectal septum malformation sequence) among women with diabetes (Table 3).

Among pregnancies in women without diabetes, the rate of non-chromosomal anomaly was significantly higher in males (RR 1.2 [95% CI 1.1, 1.2]). This sex difference was not apparent among pregnancies in women with diabetes (RR 0.9 [95% CI 0.6, 1.2] for males vs females), although the risk ratio did not differ significantly from that observed in the general population.

Predictors of non-chromosomal congenital anomalies in women with diabetes

Peri-conception HbA1c and presence of pre-pregnancy nephropathy were significant independent predictors of congenital anomaly (Table 4). For each percentage (11 mmol/mol) increase in HbA1c, the odds of a pregnancy being affected by congenital anomaly increased by 30% (adjusted odds ratio (aOR) 1.3 [95% CI 1.2, 1.4]). LOWESS indicated that this was a steadily increasing effect for HbA1c values above 6.3% (45 mmol/mol) (Fig. 1 and Table 5). There was no evidence of risk reduction below this value, although there were very few cases in this range.

Table 4 Association of maternal and fetal factors with non-chromosomal congenital anomalies in offspring of women with pre-existing diabetes (results of univariate and multivariate logistic regression)
Fig. 1
figure 1

Association between peri-conception HbA1c in women with pre-existing diabetes and the risk (with 95% CIs) of a pregnancy affected by major congenital anomaly. To convert values for HbA1c in % into mmol/mol, subtract 2.15 and multiply by 10.929

Table 5 Risk of a pregnancy affected by major congenital anomaly in women with pre-existing diabetes, by peri-conception HbA1c

Pre-pregnancy nephropathy was associated with greater than two-fold increased risk of congenital anomaly (aOR 2.5 [95% CI 1.1, 5.3]). Gestation at booking in weeks (aOR 1.1 [95% CI 1.0, 1.1]) and parity (aOR 1.6 [95% CI 1.0, 2.5]) were also included in the final adjusted logistic regression model (p < 0.1) although the associations did not quite reach the nominated significance level (p < 0.05). Of the four variables that were retained in the adjusted model, the highest predictive contribution was attributable to HbA1c (standardised beta coefficient, β = 0.41), which was more than twice as important as parity (β = 0.19), and over 2.5 times more important than gestational age at booking (β = 0.16) and nephropathy (β = 0.15).

In univariate analysis, socioeconomic status (OR 2.0 [95% CI 1.2, 3.2]) and lack of folic acid (OR 2.0 [95% CI 1.3, 3.3]) were significant predictors of pregnancy affected by congenital anomaly. However, these effects were attenuated below significance when adjustment was made for HbA1c. There was no evidence that any of the associations between variables in the adjusted model and the risk of congenital anomalies was different in women with type 2 diabetes compared with women with type 1 diabetes.

Type and duration of diabetes, fetal sex, maternal ethnicity, early pregnancy BMI, smoking during pregnancy, pre-pregnancy retinopathy, and neuropathy were not significantly associated with the risk of congenital anomaly in either unadjusted or adjusted models.


This population-based cohort study provides robust estimates of the risk of major congenital anomaly among offspring of women with pre-existing diabetes. Overall, one in 13 singleton deliveries (7.7%) was affected, and the rate of non-chromosomal anomaly was almost four times higher than in women without pre-existing diabetes. Peri-conception HbA1c has previously been reported to be associated with congenital anomaly [4], but the association with pre-existing nephropathy is, to our knowledge, previously unreported. The risk of congenital anomaly increased linearly with increasing HbA1c above 6.3% (45 mmol/mol), by nearly 30% for each 1% (11 mmol/mol) increase.

This study linked independently and robustly ascertained congenital anomaly cases with detailed clinical information on pregnancies in women with diabetes, notified to long-standing population-based registers. This minimised potential detection bias between pregnancies in women with and without diabetes, and enabled exploration of the independent effects of a wide range of clinical and sociodemographic risk factors. Ascertainment and coding of anomalies was consistent throughout, standardised according to internationally agreed criteria, and independent of diabetes status. We restricted our analysis to EUROCAT defined major anomalies, because these are consistently ascertained, and have the greatest impact on mortality and morbidity. Pregnancies in women with diabetes are subject to increased antenatal surveillance, leading to the potential for ascertainment bias unless, as in NorCAS, cases are notified whenever diagnosed in childhood (to age 12 years). This is particularly important for cardiovascular anomalies, many of which are only diagnosed in early childhood. Most previous cohort studies of anomalies in pregnancies complicated by diabetes include only those diagnosed antenatally or apparent shortly after birth, a major methodological limitation [2, 3, 5, 1619].

This is one of the largest cohort studies to date, including 120 cases of major non-chromosomal anomaly in women with both type 1 and type 2 diabetes, and the only such study to include detailed clinical information. The north of England benefits from a long history of collaborative clinical networking within maternity and neonatal services, and the NorCAS and NorDIP surveys were initiated by pioneering clinicians in the 1980s and 1990s. The surveys are now supported by the Regional Maternity Survey Office (RMSO) which provides a focus for data collection and dissemination across a number of linked surveys of maternal and perinatal health outcome (

HbA1c was measured within three months prior to conception in nearly half of cases, and this is likely to reflect peri-conception glycaemia better than first trimester measurements. However, information on covariates such as maternal age and parity was not available for unaffected pregnancies in women without diabetes, and we were therefore unable to adjust our relative risk estimates. Few of the women with diabetes were of non-white ethnicity. Robust information about hypoglycaemic therapy was not available, so we were unable to investigate any potential association with congenital anomaly risk. The study may have lacked power to quantify the relative risk for anomalies with a small effect size, or where very few cases were reported. In the multivariate analyses, we estimated that we had adequate power to detect a medium effect size for almost all variables examined. The study may have missed some associations with smaller effect sizes.

We estimated the relative risk of non-chromosomal congenital anomaly in the offspring of women with existing diabetes to be nearly four-fold higher than the general population. Previously published estimates range from two- to threefold [2, 3, 16, 17, 20] to tenfold [21, 22]. Direct comparison with the current study is difficult due to differences in ascertainment and classification of anomalies, and lack of comparable risk estimates for offspring of women without diabetes. In a large cohort of births to women with diabetes from England, Wales and Northern Ireland (CEMACH enquiry), the prevalence of major non-chromosomal anomaly was 4.6%, compared with 7.2% in the current study. This difference may reflect the fact that CEMACH did not have access to a population-based register and only identified cases apparent within 28 days of delivery. Our study is population-based and draws on multiple sources to identify cases of anomaly diagnosed at any time up to age 12 years. Under-ascertainment is also likely to explain the CEMACH study's low reported prevalence ratio of 2.2 for congenital anomaly in women with and without diabetes, as the comparison was with age-adjusted prevalence rates from the EUROCAT network of population-based registries [2]. The current study estimated a 3.8-fold increase, based on a direct comparison of the congenital anomaly rates in women with and without diabetes from the same source population, indentified independently of diabetes status.

Only two variables, higher peri-conception HbA1c and pre-existing nephropathy, were significant independent predictors in multivariate analysis. Parity and gestational age at booking were retained in the multivariate model but the associations did not reach statistical significance. There was no evidence of an independent effect of maternal age, smoking, ethnicity and early pregnancy BMI, which have been associated with congenital anomaly risk in the general population. A higher rate of congenital anomaly was observed in women resident in more deprived areas; this was largely attributable to higher peri-conception HbA1c in these women. We found no evidence that the increased risk of anomaly in women with diabetes was specific to males, in contrast with an earlier report [23], although we confirmed the increased risk for males in the general population [24, 25]. There was no evidence that any of the identified predictors of congenital anomaly were different in type 1 and type 2 diabetes.

Peri-conception HbA1c was the most important independent predictor of congenital anomaly risk, confirming previous reports [46]. The current study identified a linear relationship with HbA1c for values between 6.3% and 11% (45 and 97 mmol/mol). The odds were lowest for HbA1c = 6.3% (45 mmol/mol), although still above background population levels, and increased by approximately 2% in absolute terms for each 1% (11 mmol/mol) increase, slightly lower than previous reports [5, 6]. We found no evidence of further reduction for values below 6.3% (45 mmol/mol), although there were few individuals in this range.

Current guidance from the American Diabetes Association recommends a target HbA1c <7% (53 mmol/mol) prior to pregnancy [26]. In England, the National Institute for Health and Clinical Excellence (NICE) suggests a target for preconception HbA1c <6.1% (43 mmol/mol), if safely achievable, and strongly discourages pregnancy at levels >10% (86 mmol/mol) [27]. Our results indicate that there appears to be no specific threshold for change in congenital anomaly risk, and hence do not provide support for particular peri-conception HbA1c targets, but rather provide risk estimates across a range of HbA1c levels. Our results further suggest that even achieving near normal levels of HbA1c does not eliminate the increased risk of congenital anomaly attributable to diabetes. All women with diabetes should be encouraged to achieve as great a reduction in HbA1c as possible prior to conception.

There was a greater than twofold increased risk of congenital anomaly in the offspring of women with pre-existing nephropathy. This group is known to be at increased risk of adverse pregnancy outcome [28, 29], but this is the first study to suggest a specific increased risk of occurrence of congenital anomaly. This finding requires confirmation in other studies. Nephropathy may reflect a history of prolonged poor glycaemic control, including high variability in glucose levels, which may not be reflected by HbA1c [30]; however, neither retinopathy nor neuropathy conferred increased risks of congenital anomaly. Women with nephropathy usually require antihypertensive medication and are often treated with ACE inhibitors, which have been associated with congenital anomaly risk [31]. Current guidance suggests that these and other potentially teratogenic medications should be discontinued prior to conception [27, 32] but many pregnancies are unplanned and the extent of peri-conception exposure to potentially teratogenic medications is unknown. We were unable to investigate this issue as the registers do not record details of peri-conception medications. There is evidence for a genetic influence on diabetic nephropathy, and it is possible that an association with congenital anomaly may have a genetic basis [33]. Oxidative stress is thought to play a role in the development of nephropathy as well as in congenital anomaly [34]. These potential shared mechanisms merit further research.

Type of diabetes was not independently associated with risk of congenital anomaly, and did not modify the association with other variables. There was a slightly higher unadjusted risk of non-chromosomal anomaly among women with type 1 diabetes (RR 1.4 [95% CI 0.9, 2.2]), which may have been significant with a larger sample size; however the effect was heavily attenuated by adjustment for HbA1c, suggesting that this is the main driver for any difference in risk between type 1 and type 2. Women with type 2 diabetes had lower peri-conception HbA1c, but were less likely to attend for preconception care, and had markedly different clinical and socio-demographic characteristics compared with women with type 1 diabetes, in line with previous reports [35, 36]. Specific approaches to improve pregnancy planning in women with type 2 diabetes may be required. Reported rates of preconception folate supplementation were generally low, suggesting poor awareness among women and/or low rates of planned pregnancies.

This study confirms the association of pre-existing diabetes with a wide range of non-chromosomal anomalies affecting most major organ systems [20, 37] and with the risk of anomalies affecting multiple systems [37, 38] Cardiovascular anomalies were the most common, reflecting their high frequency in the general population, and were not proportionally more frequent in women with diabetes. However, we confirmed very high relative risks for caudal regression sequence and laterality syndrome [38, 39], suggesting a specific effect of diabetes in the aetiology of these rare anomalies.

Given the diverse range of congenital anomalies associated with maternal diabetes, mechanisms that have a general effect on early organogenesis are likely [40, 41]. Hyperglycaemia may be directly implicated through induction of oxidative stress within the embryo [42]. Disruption of specific genetic pathways in this way has been described in animal models for neural tube and cardiac outflow tract development [43].

Blood glucose levels may fluctuate widely, even in the presence of apparently ‘optimal’ HbA1c [30]. Multiple anomalies may arise from multiple episodes of hyperglycaemia during the critical windows of development for different organ systems. Hence, approaches to reducing peri-conception glucose variability using insulin pump therapy and continuous glucose monitoring may be valuable in the prevention of congenital anomaly and should be evaluated in this regard [44].


Women with diabetes remain at greatly increased risk of offspring affected by major congenital anomaly. Achieving optimal glycaemic control prior to conception remains the most important modifiable risk factor, but is unlikely to eliminate the excess risk. Guidelines emphasise the provision of specialist preconception care to improve preparation and planning for pregnancy, but uptake remains low, and women from ethnic minority groups, socially deprived areas and with type 2 diabetes are less likely to attend. Awareness of the need for preparation for pregnancy should be incorporated into the routine care of young women with diabetes. Further research is needed to evaluate new approaches to improve the number of women with diabetes who are adequately prepared for pregnancy, and to reduce sociodemographic inequalities in outcome.

We found that women with pre-existing nephropathy were at particularly high risk of congenital anomaly. These women require specific care and support to achieve a planned pregnancy with a good outcome. Further investigation of the extent and consequences of exposure to potentially teratogenic factors in these women, including medications, is required. Interventions to reduce glucose variability and anti-oxidant therapies merit further assessment of their potential to reduce congenital anomaly risk in women with diabetes.