Prenatal Tests in Pregnancy

The tests discussed below these days have an increased ability to detect defects in the developing baby. The tests therefore raise ethical, legal and emotional dilemmas for women and their families as well as practitioners involved in maternity care. It is important to recognise that not all women and their families may want to terminate the pregnancy. However they may want early diagnosis to plan care and/or treatment for the baby following birth.

Ultrasound

Most women ask or are offered an ultrasound scan in pregnancy. Depending upon the stage of pregnancy, different types of information is sought and obtained with the ultrasound. At around seven weeks of gestation rhythmic echoes from the fetal heart can be picked up and seen on the ultrasound screen. Routinely, the ultrasound is not done in the first trimester except under special circumstances. These are:

• In case of intermittent bleeding and/or pain in the abdomen, to find out whether the pregnancy is viable.
• Sometimes, to time the pregnancy when the woman is not sure of her dates (LMP).
• At times when the internal examination of a woman reveals the uterus to be bigger or smaller than her period of amenorrhoes.
• In cases where clinical evidence raises doubts about an ectopic pregnancy.
• For Nuchal Translucency to screen for a baby with Down's Syndrome and other chromosomal abnormality.

When the ultrasound test is being performed in first trimester, you are asked to report with a full bladder so that the womb is lifted up and well visualized on the screen.

Ultrasound scan in the second trimester is performed at 18-20 weeks to make a thorough examination of the anatomy (Anatomy scan). During this scan images are often recorded on a video or CD for you for free or for a small charge.

Ultrasound scans in second and third trimester may be performed for the following reasons:

• To check dates and growth
• To check position of the placenta

The Women's Health Action Trust produce a pamphlet on Ultrasound in pregnancy which highlights the advantages and disadvantages of ultrasound and the available evidence on the topic. Ask your LMC or to visit their website to order one: Click Here

Chorionic Villus Sampling

Chorionic Villus Sampling is usually done between the 10th and 11th week of pregnancy. It can be done earlier than 10 weeks however maternal tissue can be mistaken for foetal tissue at this stage. In this procedure, a small piece of tissue (10 - 40mg) is taken from the chorion frondosum through the opening at the lower end of the womb (cervical os - transvaginally) or transabdominally. When examined under microscope, this tissue, which goes on to form the placenta, reveals the presence of certain inherited disorders. The risk of miscarriage following this procedure is 1%, however in certain families known to carry a heritable disease, it is a procedure that may be considered including the risk and benefits so that an informed decision is made. At this stage, if the foetus is found to be inheriting a disease, a termination of pregnancy can be offerred to prevent morbidity and heartache.

Amniocentesis

This image has been obtained from health.allrefer.com

A sample of amniotic fluid is withdrawn from amniotic cavity through transabdominal needle using ultrasound to avoid a placenta. Amniocentesis is not done in the first trimester but carried out at 14-16 weeks when sufficient liquor amnii is present. Before 14 weeks the amniocentesis is difficult because of small amount of liquor amnii and therefore may not provide sufficient cells to study. There is 0.5% chance that amniocentesis may cause a miscarriage and long waiting period before results are available (10 days to 3 weeks) is distressing to the parents.

Maternal Serum Screening

This test is not routine in New Zealand as in other countries and parents have to pay for the test. It is often known as a double or a triple test depending on the number of markers used to determine the risk of abnormality. It is a screening test rather than a diagnostic test. The test is carried out to search for small amounts of maternal serum alpha fetoprotein (MSAFP)in maternal blood. Normal values of MSAFP are highest in early pregnancy, decreasing as pregnancy advances. The test is usually done between 15 to 17 weeks of pregnancy. Raised levels of MSAFP are seen in multiple pregnancy, foetal death, spina bifida, exomphalus, Turners Syndrome associated with cystic hygroma. Low levels of MSAFP are associated with Down's syndrome particularly if high level of human chorionic gonadotrophin (hCG) is present. If abnormal level of MSAFP is found then a ultrasound and/or amniocentesis is carried out to check the AFP level.

X-Rays

X-rays are to be avoided during the first trimester. They are potentially harmful to your growing baby and, in early pregnancy, they provide no useful information. In case an X-ray of the chest or of some other part of the body has to be taken for some other condition, then the abdomen should be carefully shielded with a lead shield.