Vajiheh Marsoosi, M.D

Associate Professor of TUMS

Shariati Hospital

Trisomic Pregnancies are associated with altered maternal

serum concentration of various feto-placental products.

Screening in the first trimester by a combination of

maternal age, fetal NT, and serum free ß-hCG and PAPP-A

identifies about 90% of trisomy 21 pregnancies for a false

positive rate of 3%.

In trisomy 21 pregnancies

maternal serum free ß-hCG

is about twice as high and

PAPP-A is reduced to about

half compared to

chromosomally normal

pregnancies.

Adjustment is necessary in the

measured free ß-hCG and PAPP-A

Each measured level is first

converted to a multiple of the

expected normal medium (MoM)

specific to the same gestation,

maternal weight, smoking status,

ethnicity and method of conception.

In Black women the PAPP-A level is about

60% higher than in White women.

In women who smoke and those

conceiving by IVF serum PAPP-A is

decreased and this could increase in false

positive rates.

In trisomy 21 pregnancies: Free ß-hCG is higher than in euploid

pregnancies and the difference between the two

is higher at 13 than at 11 weeks.

Serum PAPP-A is lower than in euploid

pregnancies and the difference the two is

higher at 11 than at 13 weeks.

The difference from euploid pregnancies in

PAPP-A at 11 weeks is greater than the

difference in ß-hCG at 13 weeks and therefore

the overall performance of biochemical

markers is better at 11 than at 13 weeks.

In trisomy 21compared to euploid pregnancies:

The difference in biochemical markers is greater at 11 than

at 13 weeks.

The difference in fetal NT is greater at 11 than at 13 weeks.

Therefore the overall performance of screening is better at

11 than at 13 weeks.

The overall performance of combined screening is better

at 11 than at 13 weeks and may be best at 10 weeks.

Ultrasound screening for fetal abnormalities is better at 12

than at 11 weeks and much better than at 10 weeks.

A good way of achieving a high performance of screening

for trisomy 21 and diagnosing major fetal defects by

ultrasound is to carry out the blood test at 10 or 11 weeks

and the ultrasound scan at 12 weeks.

Other defectsIn euploid pregnancies the average free ß-hCG is 1.0 MoM and

PAPP-A is 1.0 MoMIn chromosomal defects the values are: free ß-hCG PAPP-A Trisomy 21 2.0 0.5 Trisomy 18 0.2 0.2 Trisomy 13 0.3 0.4 Turner 1.2 0.5 Triploidy Digenic 0.2 0.1 Diandric 9.0 0.7

Other defects There are differences between the three

trisomies: Fetal NT is higher in T18 and T 13 than

T21 Serum free ß-hCG in trisomy 21 is high

whereas in trisomies18 and 13 this is low

Serum PAPP-A is lower in trisomies18 than in trisomy 21

Fetal heart rate in trisomy 13, unlike trisomies 21 and 18, is high