Pregnancy and Prenatal Tests

Some prenatal tests are routinely performed on all pregnant women, while others are performed in special cases or upon request. Some are only performed once, while others may be performed multiple times.

Prenatal testing can help you to monitor your baby's growth and development and will also inform you of any possible complications that your baby might be facing. Routine prenatal tests are offered to all pregnant women and are performed at each and every visit with you health care provider. These routine tests include measurements of your blood pressure, urine test and may also include a blood test.

Prior to the birth of your baby, your health care professional may recommend one or more of the following tests. Some can be carried out at any stage of your pregnancy, while others can only be performed at set times. Be sure to ask your health care provider which tests you should have to ensure your baby's health.

  Prenatal Tests Schedule Timeline Calculator

A quick guide to some of the routine tests that are used in pregnancy.

hCG and HPTs (Home Pregnancy Tests)

The most common test used to detect a pregnancy in the very early weeks uses the hormone hCG (Human Chorionic Gonadotrophin). Home pregnancy tests (HPTs) only check for the hormone hCG; a hormone that is released by the cells of the fertilised ovum in pregnancy.

A pregnancy will usually only continue after implantation if menstruation is prevented. Estrogen and Progesterone are produced by the Corpus Luteum and prevent menstruation by maintaining the lining of the uterus. The corpus luteum itself is maintained by hCG that is produced by the cells of the placenta after implantation of the fertilised ovum.

Normal hCG levels vary widely between different women and in different pregnancies for the same woman. Be very careful when trying to 'interpret the numbers'. During the first 12 weeks of pregnancy, the level itself is NOT as important, as is, how much it is rising every few days.

What is the 12 week NT scan?

The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation. During the examination, the fetus is seen by abdominal ultrasound. Occasionally the view is not clear and it may be necessary to perform a vaginal scan. At the first trimester scan they confirm that the fetus is alive, they will assess the gestational age by measuring the crown-rump length and will also look for any major problems.

The CRL (crown rump length) is a universally recognized term, very useful for measuring early pregnancies. The CRL (see image above between the two white + marked 1) is highly reproducible and is the single most accurate measure of gestational age. Fetal growth charts have been developed for this purpose, but some simple rules of thumb can also be effectively used. From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.

Accuracy of CRL after 12 weeks in predicting gestational age diminishes and is replaced by BPD (biparietal diameter), a measurement of the width of the fetal head.

What is the Nuchal Translucency?

The nuchal translucency (also spelled nucal translucency) is a collection of fluid beneath the fetal skin in the region of the fetal neck (see image above between the two white + marked 2) and this is present and seen in all fetuses in early pregnancy. Currently the most accurate non invasive test for detecting a risk of Downs syndrome during pregnancy is the measurement of the nuchal translucency with an ultrasound between 11 to 14 weeks of pregnancy. 95% of measurements will indicate a reduced risk.

What is the first trimester biochemistry (blood test) for?

As part of the NT Scan a small sample (about 5 mL) of blood is collected from the expectant mother. This is to perform two biochemical tests on hormones released by the body during pregnancy on a biochemistry analyser. These are namely PAPP–A (Pregnancy Associated Plasma Protein–A) and free beta–hCG (human chorionic gonadotropin hormone).

The two hormones measured in your blood are both produced by the placenta and are unique to each pregnancy. If the two hormone levels are favourable they will reduce the chance of your baby having Down's syndrome. If they are less favourable they will increase the chance. Adding in the blood test to the NT scan, increases the overall accuracy of the test.

What is CVS (Chorionic Villus Sampling) ?

CVS is a specialized alternative test to amniocentesis. It involves removing a small amount of tissue called the chorionic villi, which is located on the outside of the fetal gestational sac and will later become the placenta. The chorion is a fetal tissue, and shares its genetic makeup with the fetus, not the mother. The chorionic villi cells may be used for chromosome analysis or other genetic testing. The chorionic villi cannot be used to test for open neural tube defects.

Second Trimester biochemistry (blood test)

The tests measure various 'markers' in your blood:

Triple Screen:
• – AFP (alpha-fetoprotein)
• – hCG (human chorionic gonadotropin)
• – uE3 (unconjugated estriol)

Quadruple Screen:
• – same as above with the addition of inhibin A

What is Amniocentesis ?

The amniocentesis procedure involves guiding a thin needle through the mother's abdomen and into the amniotic sac. Ultrasound is used to determine the location of the fetus and the best place to withdraw the fluid. The amniotic fluid contains cells that have been shed by the fetus during normal development. These cells may then be used for chromosome tests and/or specific genetic tests. The fluid itself is tested for the level of alpha feto-protein (AFP) or for biochemical genetic disorders, if appropriate.

Anatomy Ultrasound

Anatomy Ultrasound scans are also known by other names like, Level II, dating, anomaly or targeted scan.

A scan is performed at 18 to 20 weeks when the fetus is large enough for an accurate survey of the fetal anatomy, and when dates and growth can also be assessed. This is a detailed scan during which each part of the fetal body is examined. After checking your baby's heartbeat a thorough examination of your baby's brain, heart, spine, kidneys, organs and limbs will be undertaken. The placenta will be checked for its position and measurements of your baby will be done to ensure it is growing normally.

What is a glucose tolerance test ?

A glucose tolerance test, usually conducted in the 24 to 28 weeks of pregnancy, measures levels of sugar (glucose) in the mother's blood. Abnormal glucose levels may indicate gestational diabetes.
 
How is a glucose tolerance test performed?
Although the specific details of each procedure may vary slightly, generally, a glucose tolerance test follows this process:

• Mother-to-be may be asked to only drink water on the day the glucose tolerance test is given.
• An initial fasting sample of blood is drawn from a vein.
• You will be given a special glucose solution to drink.
• Blood will be drawn several times over the course of several hours to measure the glucose levels in your body.

Rhesus factor - (RhD) Negative

During your pregnancy your blood will be tested to see whether you are Rhesus positive (which means that you do have Rhesus factor) or Rhesus negative (which means that you don't).

Rhesus factor isn't a problem unless you are negative and your baby is positive. In this situation, if a few of your baby's blood cells get into your circulation during pregnancy or at birth, your body will recognise that they have something which your own blood cells haven't (ie, the Rhesus factor). Your body will then treat your baby's blood cells as invaders and manufacture antibodies to destroy them.

Your blood can be checked during the pregnancy to ensure you have not already developed these antibodies and to protect future pregnacies you will be given injections called Anti-D which will stop your body producing antibodies. After the birth a sample of the baby's blood would be taken from the cord and if the baby is found to be Rhesus Positive another injection of Anti-D will be given to you.

Group B Streptococcus (GBS)

Group B Streptococcus (GBS) are bacteria found in the lower genital tract of about 25 percent of all women. GBS infection usually causes no problems in women before pregnancy, but can cause serious illness in the mother during pregnancy and the newborn after delivery. GBS may cause chorioamnionitis (a severe infection of the placental tissues) and postpartum infection. Urinary tract infections caused by GBS can lead to preterm labor and birth.
 
GBS is the most common cause of life-threatening infections in newborns, including pneumonia and meningitis. Newborn babies contract the infection during pregnancy or from the mother's genital tract during labor and delivery.
 
The Centers for Disease Control and Prevention (CDC) recommends screening of all pregnant women for vaginal and rectal group B strep colonization between 35 to 37 weeks gestation. Treatment of mothers with certain risk factors or positive cultures is important to reduce the risk of transmission of GBS to the baby. Babies whose mothers receive antibiotic treatment for a positive GBS test are 20 times less likely to develop the disease than those without treatment.