Overview of Specialist Scans and Tests
Early pregnancy scan at 6 to 10 weeks
The aim of early pregnancy scans is to determine viability, the location of the pregnancy (intrauterine or ectopic) and the number of embryos. A heartbeat is usually confirmed from about 7-8 weeks onwards. In cases of spontaneous conception the size of the embryo will confirm the number of weeks of your pregnancy (dating of pregnancy). This scan is recommended to women who are experiencing pain or bleeding in the pregnancy, those who have had previous miscarriages or ectopic pregnancies or as a reassurance.
First trimester screening and nuchal translucency scan at 11 to 14 weeks
The most accurate dating of a spontaneous pregnancy is between 11 and 14 weeks. Usually at that stage the baby is already developed and a head, body, extremities and main organs such as brain, heart and kidneys can already be visualised and screened for major defects. The thickness of the skin fold on the back of the baby’s neck is called nuchal translucency (NT) and it is a marker for Down syndrome, other genetic conditions or heart abnormalities. The measurement is combined with the concentration values of two hormones in your blood produced by the placenta (HCG and PAPP-A) and the results are recorded as a combined first trimester screening test. If you have already had a non-invasive prenatal testing such as Harmony test showing a low risk result, you may not require the full combined screening test. However, the measurement of the NT is still recommended in order to screen for other abnormalities beyond the screening covered by the non-invasive prenatal testing.
Non-invasive prenatal test – harmony test
The non-invasive prenatal test analyses cell free fetal DNA in maternal blood and gives a strong indication of whether your baby is at high or low chance of having trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome). This test is 99% accurate to identify the babies with trisomy 21, 98% for babies with trisomy 18 and 90% for babies with trisomy 13. It involves a blood sample from a vein in your arm. and does not carry any risk of miscarriage. In about 5% of cases the test does not give a result due to very low concentration of the fetal cell free DNA in the maternal blood or due to technical problems. Repeating the sample would carry a 50% chance of a failure again. If the non-invasive prenatal test /Harmony test shows that there is a low chance that the baby has trisomy 21 or 18 or 13 it is unlikely that the baby has one of these conditions (the chance is less than 1 in 10 000).
The non-invasive prenatal test /Harmony test provides information only on the three most common chromosomal abnormalities: trisomy 21,13 and 18 and would not detect other rare genetic conditions or any physical defects, such as heart or brain abnormalities. Therefore a detailed scan at 11-13 weeks as well as anomaly scan at 20 weeks is highly recommended. If the scan at 11-13 weeks shows a high nuchal translucency (more than 3.5 mm) or major structural defects, such as exomphalos, holoprosencephaly, or megacysis, you may be advised to proceed with an invasive testing such as CVS or amniocentesis in order to investigate for other rare chromosomal conditions.
Chorion Villus sampling (CVS) and Amniocentesis
Both tests involve an introduction of a needle in the maternal abdomen and therefore called invasive testing. The CVS involves the examination of placental tissue (chorionic villi) and is performed usually between 11 and 16 weeks of gestation. Amniocentesis involves the examination of cells in the fluid from around the fetus (amniotic fluid). The test is safe to be performed after 16 weeks gestation.
Both CVS and amniocentesis are diagnostic tests. The risk of miscarriage due to CVS or amniocentesis is negligible. If you were to miscarry due to the test, this would happen within the next five days.
Anomaly / Morphology scan at 16 to 23 weeks
The purpose of the anomaly scan is a detailed assessment of the anatomy of your baby in order to determine the presence of congenital defects such as spina bifida, heart or brain abnormalities. The morphology scan includes assessment of fetal anatomy and growth, position of the placenta, and amount of amniotic fluid. Due to the development of the baby’s brain, and the size and complexity of the baby’s heart, 20 weeks onwards is the optimum gestation to perform the routine anomaly scan. However, most of the major abnormalities are visible from 16 weeks and therefore an early anomaly scan at 16-18 weeks may be recommended on special occasions or simply for reassurance. Wellbeing Scan, Growth and Doppler Scan at 24-40 weeks
This scan includes the measurement of the size of the fetal head, abdomen and thigh bone and calculation of an estimate of fetal weight as well as the position and maturity of the placenta, the amount of amniotic fluid, the movements of your baby and the blood flow from the placenta towards your baby. The estimated fetal weight gives an indication to the size your baby will be at birth and also could identify too small or too large babies for that gestational age. The proposed timings for growth scans are at 28, 32 and 36 weeks, however, they may vary depending on the fetal growth velocity and the presence of pregnancy complications such as high blood pressure or diabetes.
Cervical length scan
Measuring the length of the cervix during pregnancy can help identify if you’re at risk of having a premature delivery. The scan is performed internally through the vagina. If your cervix measures less than 25 mm you may be at higher risk of having your baby too early and a cervical stitch or progesterone treatment may be recommended. This scan is usually performed at around 20 weeks or in the first trimester if you have already experienced an early delivery, or if you have had a procedure performed on your cervix, such as LLETZ.
3D and 4D scans
This type of scan has a limitation depending on the fetal position and the amount of amniotic fluid. The best time of obtaining optimal images of the face of your baby is between 30-34 weeks.