Interstitial pregnancy undetected during earlier first-trimester screening for fetal aneuploidy at 13 weeks' gestation.

First-trimester screening for fetal aneuploidy using nuchal translucency (NT), pregnancy-associated plasma protein A, free or total beta-hCG, and maternal age constitutes a very effective screening test for fetal Down syndrome. We describe a case in which a patient presented at 14 weeks' gestation with an acute abdomen 1 week after first-trimester screening (including NT measurement) performed elsewhere, which was negative for trisomies 21 and 18. Sonographic examination revealed an interstitial pregnancy with a singleton fetus with present cardiac activity, which had not been noted 1 week earlier at the time of earlier transabdominal NT measurement. This case indicates that successful acquisition of a NT measurement during first-trimester screening for fetal aneuploidy does not negate the rare possibility of an unusual ectopic pregnancy.