Estimating the proportion of all observed birth defects occurring in pregnancies terminated by a second-trimester abortion.

Background: Prenatal termination of pregnancy may underestimate risks or cause bias in epidemiological studies of birth defects if such studies measure only defects diagnosed postnatally. We aimed to estimate the proportion of all fetuses with birth defects terminated in the second trimester of pregnancy-overall and for specific defects.

Methods: The study comprised all pregnancies ending in a singleton birth, miscarriage, or termination of pregnancy for which health care services were sought, as recorded in Danish medical registries between 1 January 2007 and 31 December 2011.

Results: Of the 420,090 pregnancies, 307,637 fetuses survived until gestational week 12 or beyond; of these, 296,373 (96%) ended in a live birth or stillbirth and 11,264 (4%) ended in a second-trimester termination. The prevalence of birth defects among live births and stillbirths was 3% (9,790/296,373); the corresponding prevalence among second-trimester-terminated pregnancies was 14% (1,563/11,264). Although only 4% of all pregnancies ended in a second-trimester termination, 14% (1,563/11,353) of pregnancies with birth defects were ended by a second-trimester termination. The groups of birth defects with the highest proportion of second-trimester terminations were defects of the nervous system (347/740; 48%) and abdominal wall (58/149; 39%). For many types of birth defects, however, that proportion was less than 10%.

Conclusion: The proportion of terminated pregnancies carrying birth defects is considerably greater than the corresponding proportion for pregnancies that end as live births or stillbirths. The proportion of birth defects unobserved at birth due to second-trimester terminations depends on type of defect and lethality.