All pregnant women are offered screening tests, such as scans and/or blood tests, to assess the chance of a chromosomal abnormality in the baby. These tests are non-invasive and so do not have any risk to the baby. Screening tests can indicate if a pregnancy is in a low or high risk category, but do not give a definite result.
Continual improvements in antenatal screening mean that you must ask your midwife or obstetrician for up-to-date information about new tests that may be available. Some couples choose not to have any tests.
SOFT UK would like to acknowledge the valuable contribution from Dr Lucy Kean Consultant Obstetrician and Subspecialist in Fetal Medicine, and Lorna Parsons Midwife, of the Fetal Care Unit, Nottingham University Hospitals, City Campus.
1ST TRIMESTER COMBINED SCREENING
The National Screening Committee recommends that this should be offered as the preferred early screening test. It is performed between 11 and 13 weeks of pregnancy and uses ultrasound Nuchal Translucency (NT) measurement, plus a blood test to measure free beta hCG and PAPP-A.
MATERNAL SERUM SCREENING TEST (TRIPLE OR QUADRUPLE TEST)
This is a blood test carried out between 15 to 20 weeks of pregnancy. Raised or lowered levels of certain chemicals in the blood can indicate an increased risk of Down syndrome.
All pregnant women are usually offered two ultrasound scans. Ultrasound scans cannot diagnose a chromosome abnormality but can look for structural abnormalities, (for example, a heart defect orholoprosencephaly), and can look for chromosomal markers. Thesemarkers, or 'clue signs', are signs that can be seen in normal babies, but are more common in babies with a chromosome problem. Abnormalities see non scan, or a cluster of chromosomal markers may alert medical staff to suspect a chromosome problem, but such a diagnosis can only be confirmed by an invasive diagnostic test.
The first scan is called an early dating scan and it usually happens after 8 weeks of pregnancy.
The purpose of the dating scan is:
- To find out how many weeks pregnant you are
- To check whether you are expecting more than one baby
- To measure the size of the fluid area at the back of the baby's neck (this area is called the nuchal translucency or NT)
- To check that the baby is growing in the right place
- To check your baby's development
Some abnormalities may also be detected at this scan such as neural tube defects, for example spina bifida.
MID TRIMESTER ANOMALY SCAN
The second ultrasound scan is often called a mid-trimester anomaly scanand it usually happens between 18 - 20 weeks of the pregnancy and screens for major structural anomalies. The objective of this scan is to:
- Offer choice to women and their partners about their screeningoptions and pregnancy.
- Identify serious abnormalities at a time when choices can be madewhether to continue or terminate a pregnancy.
- Identify abnormalities which may benefit from or need earlytreatment following delivery.
Before you agree to undergo this scan, you should be aware that the aim of this scan is to check your baby's development and that problems with your baby may be found. It is important to note that not all abnormalities can be seen on an ultrasound scan; just because nothing is picked up onthe scan does not mean that your baby will have no problems or anomalies. There are things that the ultrasound scan cannot pick up. If the sonographer thinks that there may be a problem with your baby, you may be offered a further scan or an invasive diagnostic test such aschorionic villus sampling or amniocentesis. These tests look at your baby's chromosomes and can confirm whether he/she has a certain condition or not.
'Following a routine scan there were doubts concerning my baby and I was referred for more detailed tests'
'They could not see all four chambers of the baby's heart because of the way the baby was facing so one week later I was scanned again. This took over an hour and I was beginning to get worried. This time the Consultant had a Paediatrician with him and he told me my baby had a cleft palate,a possible cleft lip, and the heart and kidneys were showing up as brightareas on the scan.'
'Early scans in my pregnancy showed no problems. Suspicions were aroused when a cleft lip showed up on a routine scan at 19 weeks. A detailed scan was done immediately and showed a heart defect.'
Many babies with trisomy 13/18 have heart defects and specialist centrescan perform a detailed scan of the baby's heart during pregnancy.