Objective Trisomy 18 (T18) and trisomy 13 (T13) are the second and third commonest aneuploidy syndromes, with prevalences of approximately 4:10,000 (T18) and 2:10,000 (T13) births. We aimed to examine the natural history (including diagnosis, pregnancy outcome, complications and survival) of these pregnancies in a setting where termination of pregnancy for fetal abnormality is illegal.

Study design A retrospective review was performed of confirmed cases of trisomies 18 and 13 from 2001 to 2012. Following case identification, individual charts were examined.

Results 46 trisomy 18 and 24 trisomy 13 pregnancies were identified. Median maternal age was 38 years (T18) and 35 years (T13). Most T18 cases (74%) were diagnosed prenatally, however, less than half (45%) of T13 cases were prenatally diagnosed. 36% (T18) and 18% (T13) of live born fetuses were delivered by emergency Caesarean section, the commonest indication being distress in undiagnosed fetuses. All but three T18 pregnancies and one T13 pregnancy continued to a natural outcome. 48% (T18) and 46% (T13) survived following birth, for a median of 1.5 days (T18) and 7 days (T13). A significantly longer survival time was found in T18 female infants compared with males. One T13 infant is alive at over one year of age.

Conclusions This study provides information for professionals and patients regarding the natural histories of trisomies 18 and 13 and is one of the largest series available. These pregnancies can go undiagnosed antenatally if anomaly screening is not undertaken. While many die in-utero, postnatal survival is also possible.