First Trimester

Risk Assessment

11w - 13w week scan

NT scan/Down syndrome screening

This scan can be done between 11 weeks 4 days to 13 weeks 6 days. During the examination we establish the type of pregnancy (singleton or twins), accurately date for gestational age, screen for structural normality and chromosomal abnormalities (including Down, Edward and Patau syndrome) as well as identify women at increased risk for developing pre-eclampsia and fetal growth restriction.

Structure which we measure/evaluate During screening for chromosomal abnormalities are:

If you wish, one can also determine the gender at this stage. It remains the choice of the parents how much information they would like to obtain about their pregnancy. The scan is usually performed transabdominally, however in some cases it might be necessary to use a transvaginal approach. 

Your individual risk for the pregnancy is calculated after the ultrasound examination by combining your maternal age, ultrasound findings and biochemistry (blood sample taken between 8 – 10 weeks gestation which measures the placental products free β-hCG and PAPP-A).

This is a screening technique (no needles involved) which has a 95 % sensitivity with a 5 % false positive when done by a qualified health care provider. To ensure a screening test with a high sensitivity, it is important to do the first-trimester blood test between 8 and 10 weeks of pregnancy, thus ensuring little false positive and negative results. If you have not already received a request form from your Obstetricians, GP or Midwife, please contact them for one. Please remember to bring your doctor’s referral letter along. 


Why is this combined method of first-trimester screening important? To reduce the number of women requiring invasive testing from about 20 % to less than 3 % and to early diagnose major fetal abnormalities. At the same time increasing the detection rate of Down’s syndrome and other major chromosomal abnormalities from less than 50 % to more than 95 %.

It is important that an accredited health care professional (might it be a Sonographer, Sonologist, Obstetrician or Fetal Medicine specialist) conduct the first trimester screening. To avoid disappointment, please confirm in advance on the Fetal Medicine Foundation (FMF) if the person is accredited.


Other options of screening are tabulated below. Please note that not all carry a high sensitivity for detecting fetuses with Down syndrome.

Method of screening Proportion of total population Proportion of all cases of T21
Maternal age 5% / 20% 30% / 50%
Second trimester serum biochemistry 5% 50-70%
Age, NT, FHR, β-hCG, PAPP-A (combined) 3-5% 90%
Combined plus additional ultrasound markers 2.5% 95%
Cell free DNA in maternal blood <0.1% >99%

courtesy of FMF