Alphafetoprotein levels in amniotic fluid from 8 to 18 weeks of pregnancy.

OBJECTIVE

The aim of this study was to ascertain the normal range of amniotic fluid alphafetoprotein (AFP) levels from 8 to 18 weeks of pregnancy.

DESIGN

Amniotic fluid samples obtained by amniocentesis were analysed by radioimmunoassay for AFP. From 8 to 12 weeks, fluid was obtained by transvaginal amniocentesis prior to termination of pregnancy. Samples from 13 to 18 weeks were obtained from amniotic fluid specimens submitted for cytogenetic analysis.

SETTING

A tertiary referral prenatal diagnostic service.

SUBJECTS

Two hundred thirty-seven women between 8 and 18 weeks gestation.

RESULTS

High levels of AFP were present in amniotic fluid at 8 weeks (median 83.0 kU/ml), falling rapidly to 11 weeks (median 19.9 kU/ml) (P < 0.01). Levels then rose to a peak at 13 weeks (median 30.7 kU/ml), thereafter falling to 18 weeks.

CONCLUSIONS

A single coherent view of the levels of AFP from 8 to 18 weeks of pregnancy is presented for the first time. The rapid changes and complex pattern of AFP levels in early pregnancy suggest that interpretation of amniotic fluid AFP levels in the diagnosis of neural tube defects may be more difficult between 8 and 13 weeks of pregnancy than in the second trimester.