The 12 weeks ultrasound examination can be done from 11 weeks 4 days to 13 weeks 6 days. The scan is usually performed transabdominally. Sometimes, it may be necessary to do the scan transvaginally.
The aims of the 12 weeks ultrasound are:
* To date the pregnancy accurately (if this has not been done yet).
* To diagnose the type of twin (or other multiple) pregnancy. It is important to know whether each twin has its own placenta, or whether they share a common placenta. If they share a placenta, it is advisable to monitor the pregnancy more closely.
* To check whether the baby is growing and developing normally. Some major abnormalities can be visible at 12 weeks, but it is much better to have an ultrasound examination at 20 -22 weeks as well to exclude structural abnormalities as far as possible.
* To assess the risks of Down’s syndrome and other chromosomal abnormalities.
* To assess the risk of developing pre-eclampsia (high blood pressure which is caused or aggravated by pregnancy).
Your individual risk for this pregnancy is calculated by taking into account your age, measurement of hormones in your blood and the ultrasound findings.
Personal risk of Down syndrome
Most babies are normal. On the other hand, each woman, regardless her age, has a small risk of having a baby with a physical or mental handicap.
The only way of knowing for certain whether a baby has a chromosomal anomaly or not, is to do an invasive procedure such as a chorionic villus sample or amniocentesis. These procedures have a risk of about 1/300 to cause a miscarriage. The most accurate, but most expensive, screening test for Down syndrome is non-invasive testing of fetal DNA.
The ultrasound examination between 11 and 13 weeks also evaluates the risk of Down syndrome, which depends on:
* your age,
* the level of two hormones (free ß-hCG and PAPP-A) in your blood and
* ultrasound findings: especially the thickness of fluid behind the baby’s neck (nuchal translucency thickness) and possible structural anomalies. To refine the risk assessment, the nasal bone, heart rate and blood flow in a vein between the umbilical cord can also be evaluated.
After the ultrasound evaluation, the risk of Down syndrome and other chromosomal anomalies (trisomy 13 and 18) will be discussed with you. Based on the risk assessment you can decide whether you would like invasive testing (by mean of a chorionic villus sample or amniocentesis) (usually if the risk is higher than 1:300) or non-invasive DNA testing.
The risk assessment remains a screening test. Under optimal circumstances the test would detect 90% of babies with Down syndrome, with a 5% chance of a false positive test. Optimal conditions include a blood test done between 8 to 10 weeks and an ultrasound performed by someone with the specific training and whose ultrasound examinations are subject to independent quality control. (Although the blood tests are done previously, their results are only taken into account once the ultrasound results are available.)
The blood tests are most accurate when done between 8 and 10 weeks (although they can be done until 14 weeks). Including the blood tests improve the overall accuracy of screening for Down syndrome.
An ultrasound at 20 weeks is also recommended to exclude physical problems as far as possible.
Personal risk of pre-eclampsia
Pre-eclampsia is a dreaded condition in pregnancy: high blood pressure brought on by the pregnancy, which is of danger to yourself and the baby. The risk can be calculated in the same way as calculating the risk of Down syndrome based on your background risk, the results of hormone tests and physical and ultrasound examination. 90% of early pre-eclampsia (developing before 34 weeks) can be detected (for a 10% false positive risk), and 80% of early pre-eclampsia prevented by using low-dose aspirin and calcium supplements. As with the Down syndrome screening, the most accurate results are obtained by practitioners with the right training and accreditation.
Depending on the practice, your husband or partner is very welcome to join you during the ultrasound scan. Another friend or your parents (or in-laws) may also be allowed, depending on the individual practice. Keep in mind, though, that too many people can make it difficult for you and your partner to focus if something specific needs to be explained. Make sure beforehand who can accompany you to the scan, and whether the sonographer would prefer you arriving with a full or an empty bladder.