EUROCAT subgroups, based on ICD9‐BPA and ICD10‐RCPCH codes: (https://eu‐rd‐platform.jrc.ec.europa.eu/sites/default/files/Section%203.3‐%2027_Oct2016.pdf). Well known teratogenic exposures (diabetes, valproic acid, thalidomide, insufficient folic acid) result in specific congenital anomalies, not all congenital anomalies. This is mainly related to the mechanism of action, but different defects may also result from different timings of exposure during organogenesis. Studies that combine all congenital anomalies together may not detect increases in risk of specific anomalies, and it is recommended that as sample size increases, more specific congenital anomaly groups should be analysed. In order to aid meta‐analysis across studies, results should always be disaggregated by specific CA subgroups to as great an extent as possible. EUROCAT subgroups allow this to be done in a standardised manner.

Minor anomalies for exclusion. These are frequent anomalies, inconsistently diagnosed, with little medical or functional significance, which can cause considerable “statistical noise” in prevalence rates unless standard exclusions apply. Some are not true congenital anomalies e.g. patent ductus arteriosus in preterm births. Two very similar lists are published by EUROCAT (https://eu‐rd‐platform.jrc.ec.europa.eu/sites/default/files/Section%203.3‐%2027_Oct2016.pdf) and the WHO 9789240015395‐eng.pdf (who.int)

EUROCAT guidance for calculating Total Prevalence rates https://eu‐rd‐platform.jrc.ec.europa.eu/sites/default/files/Section%204.1‐%2027_Oct2016.pdf CA cases should include livebirths, fetal deaths from 20 weeks (or a suitable threshold gestational age after which diagnosis is well recorded) and TOPFA (i.e. termination of pregnancy for fetal anomaly after prenatal diagnosis). The denominator for total prevalence rates is all births (live and still). TOPFA can be included in the denominator but are too few in relation to births to make a difference (TOPFA may account for up to 1 for every 100 births in Europe, usually less). Lack of TOPFA in the numerator underestimates the risk of CA. Non‐TOPFA terminations and spontaneous abortions should not be included in either the numerator or the denominator. This is because they are incompletely reported, and incompletely examined for presence of a CA, and being numerous can considerably bias CA prevalence downwards.

EUROCAT Prevalence rates of major congenital anomalies https://eu‐rd‐platform.jrc.ec.europa.eu/eurocat/eurocat‐data/prevalence_en Prevalence rates per 10000 births of 92 subgroups per EUROCAT registry, per year, per type of birth. Can be used as external comparator rates, or to help in sample size calculations when planning studies, or to help interpret how unusual the distribution of anomaly types in a case series is.

ConcePTION Core evidence elements for generating medication safety evidence for pregnancy using population‐based data contains extensive information on Congenital Anomalies, available at ConcePTION‐D1.2.pdf (imi‐conception.eu)

CA Registry Data Quality indicators. Congenital anomaly registries measure the quality of their data, and data quality indicators help to decide which data should be included that meets quality standards. EUROCAT Data Quality Indicators can be found at https://eu‐rd‐platform.jrc.ec.europa.eu/eurocat/data‐collection/data‐quality_en). A set of indicators suitable for registries worldwide has been published by the WHO in collaboration with ICBDSR 9789240015395‐eng.pdf (who.int)

Pictorial Guides to Congenital Anomalies to aid in identification and coding. In the high income countries of Europe, health systems generate good quality data on CA, and the challenge for the registry is to access the records (increasingly in electronic form), and to cross‐check between different stages of the baby’s diagnostic journey. In low to middle income countries, the availability of specialist healthcare professionals is much more patchy, and it may be difficult to collect CA data for research and surveillance. The WHO, in collaboration with CDC and ICBDSR, has issued a useful Quick Reference Handbook of selected congenital anomalies with photos and diagrams (9789240015418‐eng.pdf (who.int)). The Global Birth Defects has developed an app to help non‐experts identify birth defects with simple‐to‐use pictorial pathways (https://globalbirthdefects.tghn.org/download‐birth‐defects‐surveillance‐app/). The ECLAMC network in South America have an extensive online database of photos of congenital anomalies Home ‐ Congenital Malformations Browser (atlaseclamc.org)

Global Birth Defects website contains an extensive inventory of available resources for Congenital Anomaly Surveillance, Research, Prevention and Care Resources Inventory • Global Birth Defects (tghn.org)