Prenatal testing in the second trimester of pregnancy

Second-trimester biochem­ical markers

Measuring β-hCG, alpha-fetoprotein and oestriol in the mother’s blood serum between weeks 16 and 18 of pregnancy, combined with the mother’s age, can identify 67% of pregnancies with Down syndrome, as well as a significant proportion of trisomy 18 and 13. Adding inhibin A can improve the diagnostic sensitivity by about 7% (quadruple test).

Level II ultra­sound scan

To examine the fetus in detail and rule out serious malformations and abnormalities, the first-trimester scan alone is not enough. Every pregnant woman needs a detailed scan between weeks 20 and 23 of pregnancy. In this scan, all the organs of the fetus are checked in detail and an attempt is made to rule out congenital abnormalities of the fetus (major or minor, multiple or isolated, linked or not to a chromosomal abnormality or a genetic syndrome).
This scan can rule out about 70–80% of congenital abnormalities of the fetus. Three-dimensional ultrasound helps to improve the diagnostic sensitivity. Alongside the fetus’s anatomy, the scan also checks its growth (to rule out intrauterine growth restriction) and the environment inside the uterus (placenta, amniotic fluid, blood flow with Doppler ultrasound).

Risk of chro­mo­somal abnor­mal­i­ties by the mother’s age

Mother’s age	Risk of trisomy 21 (Down syndrome)	Risk of all trisomies
20	1 in 1,667	1 in 526
21	1 in 1,429	1 in 526
22	1 in 1,429	1 in 500
23	1 in 1,429	1 in 500
24	1 in 1,250	1 in 476
25	1 in 1,250	1 in 476
26	1 in 1,176	1 in 476
27	1 in 1,111	1 in 455
28	1 in 1,053	1 in 435
29	1 in 1,000	1 in 417
30	1 in 952	1 in 384
31	1 in 909	1 in 384
32	1 in 769	1 in 323
33	1 in 625	1 in 286
34	1 in 500	1 in 238
35	1 in 385	1 in 192
36	1 in 294	1 in 156
37	1 in 227	1 in 127
38	1 in 175	1 in 102
39	1 in 137	1 in 83
40	1 in 106	1 in 66
41	1 in 82	1 in 53
42	1 in 64	1 in 42
43	1 in 50	1 in 33
44	1 in 38	1 in 26
45	1 in 30	1 in 21
46	1 in 23	1 in 16
47	1 in 18	1 in 13
48	1 in 14	1 in 10
49	1 in 11	1 in 8