The first trimester screening (FTS) refers to the combination of fetal ultrasound examination done at 11-14 weeks and maternal blood test. The original purpose of the FTS was to screen for Down Syndrome. By screening, we mean ‘sorting out’: that is sort out fetuses that have a higher chance of Down syndrome from those that have a lower chance. Over the past two decades, world-wide experience and expertise in to this new test has increased tremendously such that in today’s practice the purpose of this first trimester screening is actually two-fold: firstly, to assess the structural normalcy of the fetus and secondly to screen for Down syndrome.
This FTS is also referred to by different names : combined screening test (indicating a combination of scan and blood), nuchal scan (indicating the weightage given to the fetal nuchal translucency in the earlier days), NT scan (abbreviated form for nuchal translucency). Since some practitioners believe that the blood test is optional (and in some cases it is not affordable or available), it is not uncommon to perform only the scan component of the FTS. In such cases, this is usually referred as the Nuchal scan or the NT scan.
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.
Combined with the test performed on the mother’s blood, this FTS detects about 90% of Down Syndrome babies.
Typically, the duration of the scan is about 20 minutes but sometimes may be extended or require more than one sitting if the fetus does not “cooperate” 🙂
The second trimester target scan is the gold standard fetal evaluation scan: we speak of five components in our “target scan” : a) assessment of fetal support system; b) assessment of fetal activity and behaviour; c) assessment of fetal size; d) assessment of fetal structure, the anomaly scan, and e) assessment of cervical support to know the risk of premature delivery.
Fetal support system consists of the placenta, umbilical cord, and to a certain extent, the amniotic fluid. Abnormalities in the appearance of the placenta may indicate the need for close follow up to see if the fetal growth (ie increase in size) is on par with the expected standards. Abnormality of the umbilical cord, typically single umbilical artery instead of two, also means we will have to follow up to exclude growth issues.
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 is reassures normal cortical development to a large extent.
Fetal Size is estimated by measurement of the baby’s head, body and thigh bone length. By combining these measurements, we will be calculating the ‘estimated’ fetal weight and compare these values to the expected standard for the gestational age.
Fetal structural assessment involves a systematic documentation of the different organs of the fetal body, both external organs, and internal organs. The protocol (standard examination) includes 3 views of the head and brain, 3 views of the spine, 3 views of the face (ears not examined routinely), 3 views of the upper limbs and lower limbs (fingers not counted routinely), 3 views of tummy organs, 6 views of the heart. 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.
When all the 5 components are normal, one can safely reassure the couple and the risk of adverse outcome is very low.
Typically, the scan takes about 30 minutes to be completed. Not uncommonly, there will be two sessions required to allow the baby to turn over and show all the sides. The target scan is usually performed between 18 and 22 weeks.
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.
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 considered to be 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.
Typically, the duration of this scan is about 15 minutes.
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 anemia, 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 wont 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 anemia polycythemia 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 The Fetal Clinic is at par with the best of fetal medicine centers. 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.