Screening Tests In Pregnancy

Screening in pregnancy help with the diagnosis of conditions that can affect you or your baby before the baby is born. This means that you may get earlier, more effective treatment, or that you can make an informed reproductive decision and can be counselled for the possible outcome.

Screening tests offered can be ultrasound scans or blood tests or a combination of both.

Screening tests do not always have an ability to give you an accurate diagnosis. They only give you the likelihood of having a certain health condition. If the screening test flags up a significant risk, you will be required to consider further testing which is aimed at ascertaining whether you or the baby are affected by the condition. 

When couples or individuals have licenced fertility treatment (IUI, IVF, ICSI, etc.) they will have carried out pre-conception screening tests to rule out a possibility that they may be carrying an infection which can affect their health and /or the health of their future baby. The mandatory tests are screening for HIV, Hepatitis B and Hepatitis C. Our unit’s policy is to test for gonorrhoea and chlamydia in addition to the above-mentioned mandatory tests.

Other screening tests such as Hepatitis A, HTLV, syphilis, CMV may be required too, as dictated by the nature of the specific fertility treatment or specific patient’s risk factors.

Couples and individuals will have also been offered pre-conception genetic screening tests according to ethnicity (Cystic Fibrosis, thalassemia, Sickle Cell, Tay–Sacks, etc.). City Fertility can also offer an expanded pre-conception battery of genetic screening tests. ‘Battery of tests’ means that our genetic lab will use a single blood sample taken from you to check if you are a carrier of a number of genetic conditions. Some of these conditions may have an effect on your health and some may have no effect on you but may have implications on your child’s health in case your partner or the person donating gametes to you is a carrier too. All of the above genetic tests can be performed before you try conceiving (pre-conception tests) or during the early stages of pregnancy (pre-natal). Pre-conception tests are aimed at finding a way of reducing the chance of conception with a baby who may be affected by the genetic condition.  Pre-natal tests are aimed at assessing the risk that the baby you are carrying is affected by the genetic condition so that you can consider having a test (amniocentesis or chorionic villus sampling) to get you an accurate diagnosis as to whether the baby is affected or not.

Pre-natal screening for chromosomal anomalies

This refers to tests carried out at the early stage of pregnancy to check if there is a significant risk that the baby may have an abnormal make-up of chromosomes. The most common example of such abnormality flagged up by such screening tests is Down syndrome (DS). Babies affected by DS have 3 copies of chromosome 21 rather than a just a pair.

Having an ultrasound to check the thickness of the back of the baby’s neck (nuchal fold) combined with a blood test which are both carried out at 12-14 weeks, is the main screening method used in majority of NHS ant-natal clinics. The test can miss very few babies who have an abnormality involving chromosomes 21, 18, 13 and X chromosome (it has good sensitivity). However, it often flags up a concern when the baby has a perfectly normal set of chromosomes (it has low specificity). Once the screening test flags up a concern the next step is to have a diagnostic test which will ascertain if the baby is affected or not.  In view of this there are fair few invasive diagnostic tests (chorionic villus sampling or amniocentesis) carried out to confirm the diagnosis when the baby has a completely normal set of chromosomes.

NIPT- Non-invasive prenatal testing – During pregnancy the fetus generates genetic material (DNA) through the placenta into the maternal blood circulation. Advances in lab technologies allow us to separate and use this small quantities of fetal genetic material in the maternal circulation for the purpose of pre-natal genetic screening. Compared to the ‘standard’ screening test carried out at 12-14 weeks as described above, the NIPT has several advantages:

The NIPT  test can be performed from 10 weeks of pregnancy, two weeks earlier than the NHS 12-14 week screening.

The test has very good sensitivity and a much better specificity – unlike the combined screening test carried out at 12-14 weeks if the NIPT flags up a concern that the baby is affected by an abnormality there is only a minimal chance that the baby has a completely normal set of chromosomes. This reduces the number of unnecessary invasive diagnostic tests.

Due to its improved specificity it is also less likely to cause untoward anxiety.

While the combined screening test requires the skills of a sonographer who is experienced in nuchal fold measurements, NIPT is less operator dependent.

There are 2 types of tests you can opt for.

The STANDARD test

The standard test aimed at finding out how likely is your baby to have Down’s syndrome (involving chromosome 21), Edward’s syndrome (involving chromosome 18) and Patau’s Syndrome (involving chromosome 13). The test also checks for any sex chromosomal abnormalities. The above conditions are the most common chromosome anomalies found in new-borns. There are many other possible chromosomal anomalies but they are more likely to result in pregnancy loss than to be found in live-births. The value of this test as a screening method for the above mentioned conditions has been validated.

The EXTENDED TEST 

The extended test will give you the above validated screening for anomalies involving the sex chromosomes and chromosomes 21, 18 and 13, as well as a risk assessment of abnormalities involving all other chromosomes. In addition, the test looks for a possibility that the baby is affected by 5 types of microdeletions (deletions of a very small part of a chromosome) associated with major genetic syndromes. Yet, the validation of the test as a screening method for chromosomes other than the sex chromosomes, 21, 18 and 13 is still pending.

There are several labs providing non-invasive pre-natal genetic testing (NIPT).  NACE provided by Igenomix is currently used at City-Fertility. NACE is one of the most accurate noninvasive prenatal tests to determine the chance of the presence of three most commonly screened for chromosomal conditions in pregnancy: Down’s syndrome (T21), Edward’ syndrome (T18) or Patau's syndrome (T13).

The NACE NIPT is particularly recommended for those over 35 years of age, parents to be who have been told that there was high probability of an abnormality by other screening methods and parents to be who had a confirmed chromosomal condition in a previous pregnancy. Finally, it is for those who wish to avoid unnecessary invasive testing, which carries a risk of miscarriage.