FETAL SCANS
First trimester combined screening test
The first trimester screening (FTS) refers to the combination of fetal ultrasound examination done at 11-14 weeks and maternal blood test for Down syndrome screening. This FTS is also referred to by different names: Combined Screening Test (indicating a combination of scan and blood), NT scan test. While assessing the structural normalcy of the fetus at the 11 – 14 weeks scan, a detailed protocol checking the baby’s head, face, limbs, heart, lungs, stomach, bowel, bladder, kidneys, umbilical cord, placenta, liquor is undertaken. This protocol, followed at our centre, can reliably exclude almost all lethal malformations, and 80% of all major malformations. When the Combined Screening Test is done according to the correct standards, we will be able to
- Identify 90% of all Down syndrome fetuses
- Identify 80% of major physical defects in the fetus
- Identify 90% of mothers who would later develop severe form of Pre-
eclampsia
Targeted imaging For Fetal Anomalies (TIFFA)/ANOMALY SCAN
The second trimester target scan is the gold standard fetal evaluation scan:
- assessment of fetal support system;
- assessment of fetal activity and behaviour;
- assessment of fetal size;
- assessment of fetal structure, the anomaly scan, and
- assessment of cervical support to know the risk of premature delivery.
Fetal support system consists of the placenta, umbilical cord, and the amniotic fluid. Abnormalities in the appearance of the placenta or umbilical cord (such as single umbilical artery) may indicate the need for close follow up to see if the fetal growth is at par with the expected standards.
Fetal activity and behaviour forms an important part of the examination, since by the time of the 20-week scan there is a predictable pattern of movements in all fetuses. For example, rolling/flipping over; bending and extending the limbs; opening and closing of the hands; yawning/swallowing; breathing/hiccups; reaching out to the face etc. Normal pattern of fetal activity reassures normal brain function to a large extent.
Fetal structural assessment involves a systematic documentation of the different organs of the fetal body, both external organs, and internal organs. In the absence of any abnormality in these standard views, the chance of malformation of any of the organs is very low.
Cervical support is assessed usually by a transvaginal (internal) scan. The cervix is the neck of the womb and has an important role in maintaining the pregnancy until full term. By meticulous examination of the cervix, its appearance, and its length we will be able to predict the chance of a preterm birth or cervical insufficiency to a large extent. Appropriate treatment will be discussed with your primary physician. The target scan is usually performed between 18 and 22 weeks.
Fetal Echocardiography
In routine target scan, we examine the fetal heart using 6 views. In most instances this is adequate enough to exclude serious heart problems in the babies. However, there are instances where a specialised detailed examination of the heart is undertaken. This is known as fetal echocardiography. In fetal echocardiography, the baby’s heart is thoroughly checked for normalcy of structure and function. In addition to the usual way of examination, additionally we use Doppler mode of examination to accurately gauge function of the heart valves, the connections to and from the heart. The Fetal echocardiography, as opposed to newborn echocardiography is designed to exclude life-threatening or serious malformations in the heart. There are a few ‘defects’ that are quite common and yet may not be detected when the baby is inside the uterus. For example, hole in the heart or ventricular septal defect, may not be detected unless its large enough. Similarly, other problems with the heart valves or muscle that develop later in pregnancy may not be detected by fetal echocardiography. In summary, while fetal echocardiography is a good tool to exclude major defects of the fetal heart, there will be times when a newborn echo is required to establish normalcy or otherwise. Generally, fetal echocardiography is best performed between 22 and 24 weeks . Major indications include maternal diabetes, previous child with cardiac problem, mother with cardiac illness or any family history of cardiac defects, any suspicious finding in previous scans.
Growth Scan
The growth scan is typically done in the third trimester of pregnancy, i.e after the 28th week. The main objective of this scan is to assess the fetal size and the speed of growth (growth velocity). The scan is typically advised at 34 weeks if there are no other risk factors in the pregnancy. However, if there are any risk factors, then the monitoring usually starts at 28 weeks.
Factors that are significant to impact fetal growth include:
- Maternal diseases (heart, lungs, connective tissue disorders etc),
- A poor outcome in previous pregnancy (high blood pressure, low birth weight baby, or both),
- Abnormal blood test report in the first trimester screening or quadruple screening,
- An abnormally looking placenta in the fifth month scan, or
- The height of the uterus not corresponding to the stage of pregnancy.
While assessing for the fetal well-being, we look for fetal nutritional status and oxygen status. A well fed fetus is likely to be of appropriate weight for the stage of pregnancy and will show the correct speed of growth. The oxygen status is deduced from a combination of findings including how well the mother perceives the fetal movements, the pattern of movements observed during the scan, the fluid level around the fetus, and finally, the blood flow in the fetal brain. In addition to testing the fetal well-being, this scan is also utilised to correctly identify if the fetal head is in correct position and if the placenta is away from the neck of the womb. Fetal head position can also be easily assessed by clinical examination. Finally, we check four organs for evolving / late manifesting defects: the brain, heart, intestines, and kidneys.
Twin Surveillance
The treatment for twins is very different from that of single fetuses. All twins have increased chance of miscarriage, premature birth, stillbirth, in addition to the increased chance of maternal problems like anaemia, high blood pressure, and heavy bleeding after delivery.
During pregnancy, twins are classified as “single placenta twins” or “double placenta twins” based on how many placentas are present. This is a very important categorisation: single placenta twins have special monitoring. In most ‘double placenta twins’ (di chorionic twins), there won’t be any untoward complications and the pregnancy can be safely monitored once in 4 weeks.
However, in ‘single placenta twins’ (mono chorionic twins), there is a 15% chance of certain unique problems called ‘selective fetal growth restriction’, ‘twin to twin transfusion syndrome’, and ‘twin anaemia polycythaemia sequence’. These unique problems are due to the sharing of blood between the two fetuses through the single placenta. Timely identification of any such problems is important to intervene at the appropriate time and optimise the outcome.
The twins monitoring protocol followed at FETOSCAN is at par with the best of fetal medicine centres. All monochorionic twins are meticulously examined for size, liquor, and Doppler differences at each visit, every 2 weeks. Dichorionic twins are monitored for growth issues every 4 weeks.
It is important to know that the categorisation of twins to mono or di chorionic pregnancies is most accurate at the first trimester screening scan rather than late in pregnancy. Therefore, if your twin pregnancy has not been classified as either mono or di chorionic pregnancy after the FTS scan, it is important that you discuss this with your physician.
