The first trimester scan is carried out between 11-14 weeks.
It aims to:
- Confirm the heartbeat of the baby
- Measure the size of the baby to assess the exact week of the pregnancy and give an accurate estimated date of delivery
- Check if there is one baby or more
- Ensure that the development of the baby is normal
- Screen the pregnancy for problems in the baby and for possible problems in the mother and the baby, later on in pregnancy.
What does the first trimester scan screen for?
The first trimester scan can screen for structural problems in the baby, chromosomal problems in the baby and also for the chance for developing blood pressure related problems in the mother or growth related problems in the baby during the pregnancy.
What structural problems can the first trimester scan assess?
With advances in ultrasound technology, successful assessment of structures in the first trimester fetus is becoming possible. This allows us to confirm that the baby is developing normally with more confidence even at this early stage. However, a small proportion of fetuses will have major anomalies and this can be identified at this stage. Obviously, the extent of assessment depends significantly on the quality of the images obtained. This in turn depends on the position of the baby and how easily ultrasound waves are allowed through the maternal tissues. Also, not all structures can be assessed at this gestation, as development of structures is also dependent on the gestational age of the fetus. We would therefore recommend that this scan is always followed up with an anomaly scan at 18-23 weeks of gestation.
What happens if a structural anomaly is identified?
If a structural anomaly is identified, we will discuss it in detail with you and will arrange for further tests, scans and consultations as appropriate. A detailed plan of management will be drawn for the pregnancy and the baby including the place and time of delivery and the inclusion of a multidisciplinary team.
How does the first trimester scan screen for chromosomal anomalies?
All pregnant women have a chance related to their age for having a baby with chromosomal anomalies such as Down’s syndrome. The first trimester scan looks at the fluid behind the neck (nuchal translucency, NT), the presence or absence of ossified nasal bone (calcification process) and a few additional markers to calculate an individualized risk for patient. This risk will indicate whether the age-related chance has increased or decreased due to the scan findings. This risk is calculated particularly for common chromosomal abnormalities like Trisomy 21 (Down’s syndrome), Trisomy 13 (Patau’s syndrome) and Trisomy 18 (Edward’s syndrome). This scan is also called the nuchal translucency (NT) scan.
What is the combined screening test?
The NT scan can be combined with a blood test that estimates the level of two hormones in the maternal blood (PAPPA and free beta hCG). The findings of the NT scan are then combined with that of the blood test (levels of the hormones) to give a score that indicates the risk for the common chromosomal abnormalities like Trisomy 21 (Down’s syndrome), Trisomy 13 (Patau’s syndrome) and Trisomy 18 (Edward’s syndrome). This combination of a scan and the blood test is called the combined screening test.
What are the results I can expect from the combined screening test? How will I get them?
The results of the combined screening test will be conveyed to you by the Fetal Medicine Consultant.
Based on the calculation, your risk may be classified as “High”, “Low” or “Intermediate”.
Your consultant will discuss this in detail with you and explain the chance that has been calculated specifically for you in this pregnancy. The measurement of the nuchal translucency may sometimes be above the expected range in some babies. In some pregnancies, we may find some other markers or abnormalities. If any of these are noted, they will be explained to you at the time of the scan.
What happens if I have a high or intermediate risk?
If the risk from the combined screening falls into the high or intermediate category, your consultant will discuss it with you. You can have further tests such as Non-Invasive Prenatal Test to clarify this, if you choose to.
The results of the screening test will not be able to give you a definitive answer. This is because the combined screening test is not a diagnostic test. It is only capable of assessing the chance and can identify approximately 90-95 % of babies with Down’s syndrome. If it gives a high-risk result, it does not mean that the baby has the syndrome. In the same way, a low risk result does not guarantee a normal baby.
The need for further testing is a decision based on personal choice.
What are the types of further tests I can have for chromosomal abnormalities?
The following tests may be offered as a second line investigation after the results of the first trimester screening has been computed. However, you may choose these tests as your preferred test for screening/diagnosis for chromosomal anomalies.
The further tests could be of two types:
- Non-invasive prenatal tests (NIPT)
- Invasive tests (CVS/amniocentesis).
Please learn about these test in our next article.
How does the first trimester scan screen for pre-eclampsia and growth restriction?
At the 11-14 week scan, we can record the blood pressure of the patient and assess the blood flow to the uterus (Uterine Artery Doppler). This can be combined with some blood parameters and then a risk calculation can be made to predict the chance of developing pre-eclampsia (blood pressure problems) or growth restriction (growth problems in the baby).
The result of this screening will indicate whether you are at risk of developing these problems, later on during the pregnancy. In these situations, we would recommend that aspirin (150mg once daily) is considered until 36 weeks of gestation. This is the current recommendation as the use of aspirin is associated with a significant reduction in the risk of pre-eclampsia in the mother and/or growth restriction in the fetus. However, a small proportion of women may not benefit from aspirin and this cannot be predicted without using the tablets. Nevertheless, the screening for pre-eclampsia will make us more vigilant (if screen positive) so that the condition may be picked up early and management plans made as appropriate.
Does a normal scan ensure/guarantee a normal healthy baby/pregnancy?
No, a normal 11-14 week scan or a low risk screening report does not guarantee a normal baby or a healthy pregnancy. It indicates a lower chance for these problems as it is only a screening test.
Can the screening test harm me or my baby?
The screening test cannot harm you or your unborn baby.
Do I need to have the screening test(s)?
No, having any of these tests is entirely optional. The decision to have any test is usually made after discussion with the Consultant.
You may wish to choose to proceed with only one part of the test and decline some aspects. For example, you can have the 11-14 week scan, but decline the Down’s syndrome screening.
However, when we scan you, it is important for you to understand that the scan may pick up structural abnormalities in the baby at any stage. If any are identified, we as doctors have a duty to inform you of these problems. Your antenatal care can proceed as normal even if abnormalities are detected.
What happens if I have missed my 11-14 week scan?
You may consider other tests for screening such as the quadruple marker test, which can be done from 15-19 weeks. However, the detection rates are lower (75%) than the combined screening test described above. Alternatively, the NIPT test can be considered. The NIPT can be done from 10 weeks onwards up to any stage of the pregnancy.
Can my children come along to watch the scan?
In general, the family can come along to watch the scan. However, it is important to understand that the scan room tends to be dark and the doctor scanning you will need to focus on the scan. This might not be comfortable for young children. Further, the scan is usually straightforward and normal in most pregnancies, but it may be necessary to discuss some details in some pregnancies. All these need to be considered whilst planning to bring your family and children into the scan room.
If you have any further concerns or queries, please do not hesitate to ask your Obstetrician or Fetal Medicine Consultant before/during your scan appointment.