Effect of parity on human chorionic gonadotrophin levels and Down's syndrome screening.
OBJECTIVE
To investigate further the association between parity and human chorionic gonadotrophin (hCG) levels during the second trimester and determine whether prenatal screening for Down's syndrome might be made more efficient by adjusting for this variable.


METHODS
Measurements of hCG were analysed in relation to parity from a cohort of 16,675 singleton pregnancies with viable outcomes, and from 108 singleton pregnancies affected by Down's syndrome. Cigarette smoking during pregnancy and maternal age were also analysed in relation to hCG levels.


RESULTS
For both unaffected pregnancies and pregnancies with Down's syndrome the median hCG levels were 7% lower in parous than in nulliparous women. A dose-response relation was also documented for the unaffected pregnancies. When para 0, para 1, and para 2 women were studied separately, hCG levels rose steadily as age advanced. Adjusting hCG levels for parity led to a 0.1% reduction in variance of the distribution of hCG measurements in unaffected pregnancies and to a negligible increase in Down's syndrome screening efficiency.


CONCLUSIONS
The effect of parity is not sufficiently great to warrant routine adjustment of hCG levels as part of prenatal screening for Down's syndrome.
D000328:Adult; D000367:Age Factors; D000704:Analysis of Variance; D015415:Biomarkers / Q000097:blood; D006063:Chorionic Gonadotropin / Q000097:blood*; D004314:Down Syndrome / Q000175:diagnosis* / Q000517:prevention & control; D005260:Female; D006801:Humans; D008403:Mass Screening; D008423:Maternal Age; D009436:Neural Tube Defects / Q000175:diagnosis / Q000517:prevention & control; D010298:Parity; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011296:Prenatal Diagnosis; D012907:Smoking; D000509:alpha-Fetoproteins / Q000032:analysis
D015415:Biomarkers; D006063:Chorionic Gonadotropin; D000509:alpha-Fetoproteins