This scan is carried out from 11 weeks to 13 weeks and 6 days. The scan is usually performed trans-abdominally. The ultrasound allows a measurement to be taken of the thickness of fluid in an area behind the fetal neck. This area, known as the nuchal translucency (NT) is often larger in a fetus with Down syndrome. Measurement of the crown-rump length (CRL) of the fetus is used to calculate the gestational age of the fetus. The best time (taking into account the small size of the fetus) is having the scan between 13 weeks & 13 weeks 6 days. At this stage the fetal organs can also be examined and the gender determined in ± 70% of cases!
The NT measurement together with a maternal blood test, usually taken between 9 weeks to 13 weeks, is the most important prenatal screening test to determine the risk of the fetus having Down syndrome. (Personalised risk for Down syndrome).
The purpose of the 11-14 week scan:
1)To date the pregnancy accurately. This is particularly relevant for women who cannot recall the date of their last period, have an irregular menstrual cycle, or who have conceived whilst breastfeeding or soon after stopping the pill. We measure the size of the fetus and from this we calculate the expected date of delivery.
2)To diagnose multiple pregnancy. Approximately 2% of natural conceptions and 10% of assisted conceptions result in multiple pregnancy. Ultrasound scanning can determine if both babies are developing normally and if the babies share the same placenta which can lead to problems in the pregnancy. In such cases it would be advisable to monitor the pregnancy more closely.
3)To diagnose certain major fetal abnormalities. Some major abnormalities may be visible at this gestation, but a follow up scan at 20-22 weeks remains essential. Not all anomalies are detected at 11-14 weeks.
4)Unfortunately, in 1-2% of women who attend our clinic for the 11-14 week scan it is found that the fetus has demised, sometimes several weeks before and without any warning.
5)Risk assessment for Downs syndrome and other more rare chromosomal abnormalities (Edward syndrome, Patau syndrome, Turner Syndrome and Triploidy) with nuchal translucency measurement.
6)It is important to remember that 98% of all babies are normal.