At 12 weeks of age your baby is about 7.5 cm from top to toe (called the crown rump length), and even at this size an ultrasound is able to examine important developing structures such as the brain, heart, organs in the abdomen and bones. Ultrasound screening at this time is offered to examine these structures and although it is often known as the Down Syndrome screening test, it is so much more than that. Whilst one of the main aims of screening is for Down syndrome (trisomy 21) it may also be used to identify and two other major chromosomal abnormalities (trisomy 13 and 18) that are associated with miscarriage and late stillbirth.
There are several factors used in the screening test:
- Ultrasound measurements from you baby including its length (the crown rump length), heart rate, the thickness of the skinfold behind the neck (called the nuchal thickness), and presence of the nasal bone
- Maternal factors, such as your age and obstetric history
- Blood chemical markers produced by the placenta (collected as a blood test taken from your arm).
These separate factors are put into a mathematical formula to give you a risk of various genetic issues. It is important to remember that these tests are a ‘screening test’ so a low risk doesn’t mean no risk. If the test results reveal a high risk, further tests may be suggested to confirm your baby’s chromosome map and diagnose a specific problem.
First trimester ultrasound has also been used to screen for a variety of other pregnancy related problems including intrauterine growth restriction (IUGR) and specific problems of high blood pressure in pregnancy (preeclampsia) – both of these conditions may have a major effect on both mother and baby. A complex assessment of blood flow to the placenta, biochemical markers, past medical history for the mother and blood pressure measurement on the day will determine these risks. If your risk is considered high, a low dose aspirin treatment commencing prior to 16 weeks is recommended.