Prenatal Tests & Screening

Prenatal Tests

There are many tests that can be done throughout pregnancy to make sure that you and your baby are progressing well. Some of these tests are done routinely and others are done if there is concern about the well-being of the baby.

Routine testing

A number of tests are done routinely during pregnancy. These include urine tests, blood tests, a Pap smear, an ultrasound, and tests for gestational diabetes and Streptococcus B.

Blood tests (first medical visit)

Blood tests are done to confirm the pregnancy and to check for a number of things, including iron, infectious diseases, immunities to other infectious diseases, and rhesus (Rh) factor. Low levels of iron in the blood could mean that you have anemia, which can complicate pregnancy. Low iron levels can be treated with dietary changes and sometimes supplements. Certain infections such as HIV, syphilis, or hepatitis B are very serious and can affect the well-being of both mother and baby. Early diagnosis and treatment of these diseases can greatly improve the outcome of the pregnancy. The presence of a type of protein called Rh factor is also tested, because of the possibility of an incompatibility between the mother and baby's Rh blood groups. If there is Rh incompatibility, it can be treated with an injection of Rh immunoglobulin during pregnancy and shortly after childbirth. Women and their partners who are high-risk for certain genetic diseases such as sickle cell anemia, thalassemia, or cystic fibrosis can have a blood test done to see if they carry the traits for those conditions. If they do test positive for any of these traits, they can be referred to a genetic counsellor.

More information on:

Pap smear (first medical visit)

The Pap smear is done to check for cervical cellular abnormalities. This is a quick and usually painless, but sometimes uncomfortable, test where an instrument called a speculum is inserted into the vagina, and a swab is used to take a few cells from inside and around the cervix. The swab is sent away for investigation to make sure the cells are healthy. Sometimes women spot after the Pap smear when the site where the cells have been scraped off bleeds a bit. The Pap smear cannot harm the baby or cause a miscarriage.

Urine test (every medical visit)

A urine test can be used at the very first medical visit to confirm the pregnancy. At every medical visit, a urine test is done to measure the presence of white blood cells, sugar, and protein. White blood cells may indicate an infection; sugar could be a sign of diabetes; protein is a sign of high blood pressure called pre-eclampsia or a sign of kidney problems. Diagnosis and treatment of these conditions can help to improve the health of both mother and baby.

Ultrasound (weeks 16 to 20)

Ultrasound uses sound waves to scan the unborn baby in the uterus, and shows a video of the baby onscreen. For most couples, having an ultrasound done is very exciting because they are able to see their baby's image, albeit a bit fuzzy, for the first time. Ultrasound is used to determine the age of the unborn baby, monitor the beating heart, and check for abnormalities of the head and spine. This technique can also confirm the presence of twins and pinpoint the exact position of the placenta.

Ultrasound is usually done just once during pregnancy, between weeks 16 and 20, but sometimes it is also done late in pregnancy to make sure that the baby is growing properly. Ultrasound can be done regularly throughout the pregnancy if needed.

Gestational diabetes test (weeks 24 to 28)

It is recommended that all pregnant women be screened for gestational diabetes between weeks 24 and 28 of pregnancy. If your health care provider determines that you are high-risk for gestational diabetes, he may request the test as early as 13 weeks. In the screening test, you will be asked to drink a special high-sugar drink, and one hour later, some blood will be drawn and tested. If the screen is borderline positive for gestational diabetes, you will be asked to do an oral glucose tolerance test to confirm the condition. The oral glucose tolerance test involves fasting for four to eight hours, after which time your blood sugar will be measured. You will then be given a sugar drink and your blood sugar will be checked again two hours later. If you test positive for gestational diabetes, you will need to see an endocrinologist to help manage the disease during your pregnancy.

Streptococcus B test (weeks 35 to 37)

Group B streptococcus infections are the most common cause of life-threatening infections in newborn babies. Some doctors choose to test all pregnant mothers in their care between their 35th and 37th week of pregnancy. Any pregnant mothers who test positive for group B streptococcus are then given antibiotics when labour starts. Other doctors do not routinely test all pregnant mothers, but instead treat only those mothers who are at high risk for group B streptococcus.

The group B streptococcus test is simple and painless. The doctor will do a swab of the vagina and rectum to check for the presence of the bacteria. If the test result is positive, the woman will need to receive preventive treatment, called prophylaxis during labour. Prophylaxis treatment involves giving the woman an antibiotic during childbirth. If a mother is high risk for group B streptococcus and either was not tested or the test results have not come back, she should be treated with antibiotics.

After birth, the baby will be monitored for signs of infection and treated with antibiotics if needed.

Screening tests

A number of tests are offered to pregnant women when there is a risk or suspicion that the baby may not be developing properly. Some of these tests are screening tests, meaning that they estimate the risk of a certain abnormality developing. The following is a list of screening tests that may be offered in pregnancy.

Nuchal translucency measurement test (10 to 14 weeks)

Some mothers, for example, those over 35 years of age, are at risk of having a baby with a chromosomal disorder such as Down syndrome. In these cases, a nuchal translucency measurement test can be given in weeks 10 to 14 of pregnancy to help estimate the risk that the baby has Down syndrome. This test uses ultrasound to measure the amount of fluid that has accumulated at the back of the fetus' neck, between the skin and the underlying structures. When a fetus has a chromosomal disorder, the amount of fluid at the back of the fetus' neck tends to be increased. If this screening test shows a high risk of Down syndrome, it can be followed by a diagnostic test such as chorionic villus sampling within the first three months of pregnancy, ideally at 10 to 12 weeks gestation, or amniocentesis after week 16.

First trimester combined screening (weeks 11 to 13) followed by alpha-fetoprotein assay (week 16)

First trimester combined screening (FTS) is done in weeks 11 to 13 and consists of a combination of the nuchal translucency ultrasound and a blood test, usually done on the same day. FTS is done to estimate the chances of having a chromosomal abnormality such as Down syndrome. FTS is followed by a blood test called alpha-fetoprotein (AFP) assay in week 16. This test checks the level of AFP in the blood. AFPis a substance produced by the unborn baby's nervous system tissue. High levels of this protein could mean that the baby may have spina bifida. However, it could also mean that the pregnancy is farther along than originally thought or that the mother is carrying twins. A low level of AFP could mean that the baby has Down syndrome or simply that the pregnancy is not as far along as originally thought. Because this is a screening test, any abnormal results will be followed by a diagnostic test such as amniocentesis.

Integrated prenatal screening (weeks 11 to 13 and again at weeks 15 to 20)

This is similar to FTS followed by AFP. Integrated prenatal screening is a combination of ultrasound, nuchal translucency measurement, and two blood tests to determine your risk of having a baby with a chromosomal abnormality or neural tube defect. The ultrasound is usually done between weeks 11 to 13 of pregnancy. The first blood test is also done between weeks 11 to 13, after the ultrasound. The second blood test is done between weeks 15 to 20, the earlier the better. About four of 100 women have a 'positive' result on the integrated prenatal screening. This means that the chance of having a baby with a chromosomal abnormality or neural tube defect is higher than normal. However, most women with a positive result do not have a baby with any of these conditions. If you do have a positive result, you may choose to do a diagnostic test such as amniocentesis to determine if the baby really has one of these conditions. You may also be referred to a genetic counsellor.

Diagnostic tests

A number of tests are offered to pregnant women when there is a risk or suspicion that the baby may not be developing properly. Diagnostic tests are used to confirm the presence of a particular abnormality. The following is a list of diagnostic tests that may be offered in pregnancy.

Chorionic villus sampling (weeks 10 to 12)

Women who are at risk of having a baby with Down syndrome or other chromosomal abnormalities can have a diagnostic test called chorionic villus sampling to confirm the condition. The test is done within the first three months of pregnancy, ideally at 10 to 12 weeks gestation. Chorionic villus sampling involves the insertion of a fine tube through the cervix or abdomen into the uterus. Cells from the tissues surrounding the unborn baby can be removed and tested. Chorionic villus sampling can be done earlier in pregnancy than amniocentesis, and therefore a therapeutic abortion can be done earlier and more safely if necessary. However, there is a slightly higher risk of miscarriage with chorionic villus sampling compared with amniocentesis.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) is done in the first trimester of pregnancy around weeks 10-12. Cells that are developing into the placenta are gathered, and genetic tests are done to determine if there are any genetic abnormalities such as Down's Syndrome. CVS cannot identify whether the baby has a neural tube defect such as spina bifida because amniotic fluid is not gathered.

Amniocentesis (after week 16)

If a woman has an abnormal result on a screening test, she may be offered an amniocentesis. Amniocentesis is also offered to women over 35 because of their increased risk of having a baby with Down syndrome. An amniocentesis tests for abnormal chromosomes. In this procedure, a hollow needle is inserted through the abdomen into the uterus. Some amniotic fluid is drawn out and sent for testing. Women who choose to undergo amniocentesis should note that there is an increased risk of miscarriage with this test.


Around 16-17 weeks of pregnancy, an amniocentesis can be performed. A small needle is inserted into the amniotic sac and a sample of amniotic fluid is taken. The clinician uses an ultrasound wand on the belly, and can see on the screen exactly where the needle is placed. This fluid is tested to help determine genetic information, and to detect conditions such as Down's syndrome, spina bifida, cystic fibrosis, and sickle cell disease.

Fetoscopy (after week 16)

In this test, small incisions are made in the mother's abdomen and uterus, through which a tiny, telescope-like instrument is inserted into the amniotic sac. Fetoscopy is used to view, photograph, and take blood and tissue samples from the unborn baby. Fetoscopy is capable of detecting certain blood and skin diseases that amniocentesis cannot. However, fetoscopy poses a higher risk to the unborn baby than other techniques, and therefore it is not used very often.

Cordocentesis (after week 18)

Women at high risk may be offered this test to confirm a chromosomal abnormality. In cordocentesis, a hollow needle is inserted through the mother's abdomen into the blood vessels of the umbilical cord, close to the placenta. A sample of the baby's blood is withdrawn. Because the baby's blood vessels need to be large enough for the needle to be properly inserted, this procedure is only done after week 18 of pregnancy.

Fetal echocardiography (18 weeks)

Some women are at higher risk of having a baby with a heart defect. Fetal echocardiography is a procedure that uses ultrasound waves to study the heart of the unborn baby in great detail and to diagnose heart defects. When heart defects are diagnosed before birth, faster medical intervention can be put in place when the baby is born, which improves the baby's chances of survival after birth.

Tests in late pregnancy

Non-stress test

These tests are commonly used in late pregnancy to monitor how the baby is doing. A non-stress test might be used if the pregnancy is overdue, or if there are other potential complications in late pregnancy. In this test, the mother is hooked up to a fetal monitor, and the variation of the unborn baby's heart to certain movements is observed. If there are abnormalities in the heart's response to movement, it may indicate that the baby is in distress.

Biophysical profile

If your pregnancy is overdue or there are other potential complications, your doctor may want you to have a biophysical profile done. This involves both a nonstress test with electronic fetal heart monitoring and an ultrasound to measure the following five factors: your baby's heart rate, muscle tone, movement, breathing, and the amount of amniotic fluid. The results are scores from 0 to 2 points on each of these five measurements in a 30-minute observation period. A total score of 8 to 10 points indicates a healthy baby. A score of 6 to 8 points means you will need to be retested. A score of 4 or less may mean the baby is having problems.