First Trimester Screening

Possible accidental chromosomal anomalies are one of these risky events of pregnancy. To overcome this risk a close well defined follow up of the fetus at different stages of development has been settled over the last decades by different western gynecological associations after a lot of studies. Each trimester should benefit from a particular fetal scan beside the traditional fetal checkup done by your own gynecologist.

Three important and essential fetal scans are recommended during pregnancy. One at 12-14 weeks of gestation known as NT scan, one at 20 weeks of gestation known as morphology scan and a third one at 28 weeks of gestation known as Doppler scan. A respect of these dates is fundamental to be able to detect specific fetal features.

The first scan is the Nuchal Translucency scan (NT scan = thickness of the fluid at the back of the neck of your baby). It is performed from 12 up to 14 weeks. During this scan fetal specialist will check for fetal heart beats, total length of the baby, nuchal translucency and presence of nasal bone. The accuracy of this scan is from 60-65% only. However, to increase a little bit the accuracy of this scan we usually perform a blood test with the scan.

Two different options exist for this blood test, the first one is to do biochemical test. It is a blood sample in which we are detecting the levels of two placental hormones {Free Beta Human Chorionic Gonadotropin (FBHCG) and Pregnancy Associated Plasma Protein-A (PAPP-A)}. Both hormonal levels are used to determine the condition of the placenta. Moreover, the biochemical risk and the ultrasound risk will be used together to determine the combined risk for major common chromosomal anomalies (trisomy 21, 18 and 13). This combined risk has a sensitivity of 90-92%.

Another possible blood test during this visit is Non Invasive prenatal test (NIPT). This test will detect cell free fetal DNA fragments coming from fetal chromosomes (Chromosomes 13, 18, 21 and sex chromosomes) and circulating in maternal blood. Its sensitivity varies from 97-99%. It is considered as a screening test but has a high accuracy. It is usually recommended with advanced maternal age (above 35 years old mothers) and when mild fetal variations are detected by NT scan.

If major fetal anomalies are detected in the NT scan, an invasive prenatal test is recommended in order to obtain a piece of the placenta or some of the amniotic fluid and do fetal genetic analyses according to the ultrasound findings. 

Dr. Azza Abd El Moneim Attia Mohamed

French Board

Consultant Clinical Genetics

Feto Maternal and GenetYX Center