Medical tests in pregnancy

There are a number of medical tests that your caregiver may suggest during your pregnancy.

Blood tests

• Blood group and RH factor
• Haemoglobin level (may be repeated at the end of the pregnancy)
• Rubella antibody level
• Exposure to sexually transmitted diseases such as syphilis, hepatitis B and possibly AIDS
• Exposure to toxoplasmosis (an infection that can be acquired from contact with cat's faeces).

In later pregnancy there are other blood tests that may be done:

• Triple Test (16 - 18 weeks) - levels of hormones in the blood are measured to detect developmental abnormalities such as spina bifida and Down's syndrome. Results suggesting these conditions in the baby will be confirmed using either ultrasound or amniocentesis.

• Levels of glucose (sugar) in the blood (26 - 28 weeks) - to identify women at greater risk of developing diabetes.

• Maturation of the baby (36 - 38 weeks) - levels of oestriol in the blood can be tested over several days to determine if the placenta is functioning well, and if the baby is growing adequately. Useful in detecting small-for- dates babies and to indicate if an induction for being overdue is necessary.

Ultrasound

Ultrasound is commonly used to determine the baby’s age, and therefore due date. The accuracy of this procedure is increased if the ultrasound is done early in the pregnancy, before 20 weeks, but it still gives only an approximate date. If the baby appears to be overdue, then the due date calculated from the ultrasound should be considered together with other signs of the baby’s maturity (etermined from blood tests) and the woman’s readiness for birth, such as the ripeness of her cervix, position of the baby within the pelvis and the sensitivity of her uterus (pattern of Braxton Hicks contractions, for example).

Ultrasound is also used to confirm a diagnosis of conditions such as twins and an abnormally low-lying placenta. It is also used to confirm the results of blood tests that indicate possible abnormalities, such as Down's syndrome, spina bifida and hydrocephalus. Ultrasound can also be used to measure blood flow to the baby and to determine its sex.

Recent research shows that exposure of babies to repeated ultrasound scans may slow their growth, and may have other subtle effects, such as affecting the handedness of the baby. Until more is known about potential risks of ultrasound (especially on brain development), minimal, or no exposure may be wise. Having a scan has not been proven to improve the birth outcome for mother or baby.

The sonic aid or hand held Doppler used to listen to the baby's heart beat uses an ultrasound beam, as does electronic fetal monitoring equipment.

Tests for infection

Vaginal swabs are sometimes carried out to identify potentially harmful bacteria that may infect the baby during birth. If present, treatment with antibiotics during the pregnancy will be recommended.

Pelvimetry

An ultrasound scan (formerly done with x-rays) is taken to assess the size of the baby and pelvis if disproportion is suspected. The value of this test is questionable, since the picture obtained will provide a series of static measurements and will not show the potential capacity of the pelvis when the woman is upright and using gravity to open her pelvic bones. It is also unable to measure the potential of moulding of the baby's head as it passes through the pelvis.

Cardiotocograph (CTG)

If the woman is overdue or there is concern about the well being of the baby, the baby's responses to uterine contractions can be charted. Using external fetal monitoring equipment in the labour ward, a half-hour tracing is taken, and the movements of the baby in response to contractions (always present, but painless) can be seen. A baby who is at risk will show a slowing of its heart rate every time the uterus contracts, and this may indicate that an induction is necessary.

Kick charts

This is a simple non-invasive test. The mother records the baby's movements during a 12 hour period. She should notice at least 10 kicks in this time. A baby who is at risk will move less often than a healthy baby, with the frequency of movements usually diminishing over a period of a few days.

More invasive tests

Occasionally, the mother's health or concerns about the baby's well-being may indicate that further specialised tests be carried out. Having these tests performed is traumatic for many women, who find waiting for the results very emotionally difficult. These tests are not completely reliable, and false positive (meaning that there is a problem) or false negative (suggesting no problem) results can occur.

Before undertaking these tests, seek specialised counselling and consider carefully, in advance, what you will do once the results are known. If you are certain that you would not abort a baby that appeared to be damaged or disabled, then there may be no point in having the test done, as no action will be taken on the results and having the test will expose the baby to further risk from the procedure itself. Some parents, however, even though they will continue with a pregnancy after testing reveals a problem with the baby, are prepared to take the risks of the testing procedure itself in order to be better prepared for their baby’s arrival.

Chorionic villus sampling

This is a test for fetal abnormality sometimes offered to women in special risk groups, usually between 9 and 11 weeks of pregnancy. Using ultrasound for guidance, a small sample of tissue at the edge of the placenta (the chorion) is removed and tested to exclude abnormalities in at-risk mothers, such as Downs Syndrome, inherited disorders and sex-linked diseases such as muscular dystrophy and haemophilia.

There is a 1% risk of miscarriage with this procedure, even lower if an experienced practitioner performs the procedure. In about 10% of women , the procedure will be unsuccessful, and will have to be done again.

Results are usually available within 2 - 3 weeks so an early decision can be made about continuing the pregnancy. Termination, if desired, is easier in the first trimester.

Amniocentesis

In this procedure, the baby and placenta are located using ultrasound, then a long needle is introduced into the amniotic sac and a sample of the fluid is withdrawn.

The test is done at approximately 16 weeks to check for fetal abnormalities, including Down's syndrome and structural problems such as spina bifida. The results take 3-4 weeks to arrive, so if termination is desired it will be late in the pregnancy.

Amniocentesis can also be performed in the last weeks of pregnancy to assess fetal lung maturity and the presence of rhesus antibodies, if the mother is RH-negative.

The risk of miscarriage is about 0.5-1% in the early months, and there is a risk of injury to the baby in the later months, even using ultrasound to monitor the baby's position. The mother may also go into spontaneous labour, or continue to leak amniotic fluid.