Serum markers for Down's syndrome in women who have had in vitro fertilisation: implications for antenatal screening.

To examine the Down's syndrome screening positive rate among in vitro fertilisation (IVF) pregnancies, we measured second trimester serum marker levels in singleton IVF pregnancies (cases) and in five non-IVF pregnancies (controls) matched to each case for gestational age, age of mother, and duration of storage of the serum sample. There were 151 IVF pregnancies in which alpha fetoprotein, unconjugated oestriol (uE3), free beta-human chorionic gonadotrophin (hCG) and total hCG were measured, 104 IVF pregnancies in which free alpha-hCG was measured, and 39 IVF pregnancies in which inhibin A was measured. Median uE3 levels were 6% lower (P = 0.003), median free beta-hCG 9% higher (P = 0.024), and median total hCG 14% higher (P = 0.026) in IVF pregnancies compared with controls. The screen positive rate in the IVF pregnancies (28%) was about twice as high as that in controls (17%). High hCG levels may be explained by progesterone remaining high in IVF pregnancies. The low uE3 levels remain unexplained. In Down's syndrome screening in IVF pregnancies hCG and uE3 values should be adjusted to avoid the high screen positive rate.