Screening for Down syndrome

Figures & data

Table I. Marker levels in second trimester cases of trisomy 18 and 21.

Serum marker	T18 median MoM	T21 median MoM
AFP	0.65	0.75
Total hCG	0.32	2.06
Unconjugate estriol	0.42	0.72
Free β-hCG	0.33	2.20
Inhibin A	0.87	1.92

Table II. Changes in marker patterns in the first trimester of aneuploid pregnancies.

Aneuploidy	NT	CRL	Fetal heart rate	Free β-hCG	PAPP-A
Trisomy 21	↑2.5	↔	↑	↑2.2	↓0.5
Trisomy 18	↑3.5	↓	↓	↓0.3	↓0.5
Trisomy 13	↑2.5	↔	↑	↓0.5	↓0.3
Turner's	↑7.0	↔	↑	↔	↓0.5
Triploidy I	↑2.5	↔	↓	↑8.0	↓0.8
Triploidy II	↔	↓	↓	↓0.2	↓0.2

Table III. Maternal and pregnancy factors influencing biochemical marker concentrations.

Factor	First trimester	Second trimester
Gestational age	PAPP-A increased, free β-hCG decreased after 9 weeks	AFP, uE3 increased, hCG decreased, inhibin-A little change
Maternal weight	All decreased with increasing weight	All decreased with increasing weight, U-E3 least affected
Multiple pregnancy	Twice as high in twins – chorionicity effect	Twice as high in twins
IDDM	PAPP-A decreased	AFP decreased, U-E3 and hCG increased
Fetal sex	Free β-hCG and PAPP-A increased with a female fetus	HCG increased, AFP decreased with female fetus
Assisted conception	Free β-hCG increased PAPP-A decreased	HCG increased, U-E3 decreased
Ethnicity	Afro-Caribbean PAPP-A 50 % higher Free β-hCG higher by 10 %	AFP, hCG higher in Asian an Afro-Caribbean, inhibin-A lower in Afro-Caribbean
Smoking	PAPP-A increased	HCG, U-E3 decreased, AFP increased and Inhibin-A increased by 60 %
Gravidity/parity	Both increased as pregnancy number increases	HCG decreased
Previous pregnancy	2 to 3 times more likely to be high risk if previous was high risk	3 to 5 times more likely to be high risk if previous was high risk

Figure 1. Inverting the Pyramid of Antenatal Care to focus on the 11–13 week assessment. (modified from Nicolaides [37]).

Nicolaides KH. Turning the pyramid of prenatal care. Fetal Diagn Ther 2011;29:183–96.

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