?When you are pregnant, your doctor will do blood tests to check complete blood count, blood type, Rh factor, infections, birth defects, and more.
‌When you are pregnant, your doctor will do blood tests to check complete blood count, blood type, Rh factor, infections, birth defects, and more.

When you are pregnant, your doctor will offer tests to ensure that you and your baby get the best care possible. Some of these tests will be blood tests. Beside the standard tests, your doctor may suggest others. Parents may also request a particular type of test, perhaps based on their family history. Tests will begin in your first trimester.

A blood test is a simple way to get information about what is going on in your body. Having your blood drawn is a quick process that most people handle well. If the sight of blood makes you queasy, look away during the test. If you are nervous about needles, tell the person who is drawing your blood. Don't forget to breathe!

Routine blood tests done early in pregnancy

After your pregnancy is confirmed, your doctor will probably order three routine blood tests. These tests provide information that could be vital to your care.

Complete blood count (CBC)

This test counts the number of red blood cells, white blood cells, and platelets in your blood. You need red blood cells to carry oxygen to all of your body. White blood cells fight infection. Platelets control how your blood clots. It's unlikely that your CBC will show any serious problems, but most doctors suggest this test just in case.

Blood type

Humans have one of four blood types: A, B, AB, or O. Knowing your blood type is important in case of emergency because you can only receive blood that is compatible with your own.

Rh factor

Besides having an ABO blood type, you are also either Rh positive or Rh negative, depending upon whether you have a certain protein on your red blood cells. Your doctor will want to know your Rh classification early in your pregnancy.

If you are Rh negative and your child's father is Rh positive, your baby could be Rh positive. An Rh-negative mother carrying an Rh-positive baby can develop antibodies that could harm the baby. Doctors have many ways of dealing with this situation, called Rh incompatibility.

Tests for infections

Your doctor may also suggest tests for certain diseases and infections. Besides affecting your health, you can pass some infections to your baby. These infections may be diagnosed either with a blood test alone or in combination with other tests. Diseases that your doctor may check for include:

Special concerns about rubella

You are probably immune to rubella, also called German measles. Most people in the United States have been vaccinated for this disease. If you are unsure about your status, you can ask for a blood test for rubella.

If you are not immune and you are already pregnant, you cannot get the vaccine because it contains live virus. You should stay away from anyone with the infection. This may mean limiting travel, as rubella is common in many countries. If you are exposed, tell your doctor. Rubella can cause problems for your baby.

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First trimester screening blood tests

The term "first trimester screening" refers to a combination of ultrasound and blood tests. Doctors look at the results to determine whether you are at risk of having a baby with certain birth defects, including Down syndrome (trisomy 21) and trisomy 18.

Screening tests differ from diagnostic tests. They cannot tell you for certain whether your baby has a particular birth defect. If they show your baby is at high risk, your doctor may offer you genetic counseling and further testing.

If you are over 35 or at high risk for some other reason, your doctor may offer you another type of test. Cell-free fetal DNA screening checks your blood for samples of your baby's DNA. Once the DNA is found, it is checked for abnormal chromosomes. This type of testing, also called non-invasive prenatal testing, has a low risk for mother and baby. It is still a screening test. Results may need to be confirmed by other methods.

Optional carrier screening

You can also be tested to find out if you are a carrier of certain genetic conditions. If you are a carrier, your child's father should be checked, too. Your child will not have the problem unless the baby's father is a carrier, too.

Some people have carrier screening done before they get pregnant. If you have not been tested, you can request a test at any time during your pregnancy, including the first trimester. You can be checked for a number of conditions with a single blood test.

Experts recommend that all mothers be screened for cystic fibrosis, a serious condition that affects breathing and digestion. About one in 31 Americans is a carrier of cystic fibrosis.

Doctors also suggest carrier screening for spinal muscular atrophy (SMA), which is the most common genetic cause of infant death. They also advise testing for conditions such as sickle cell disease that affect the red blood cells.

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Medically Reviewed on 9/7/2021
References
The American College of Obstetricians and Gynecologists: "Carrier Screening," "Routine Tests During Pregnancy."

Centers for Disease Control and Prevention: "Family Health History and Planning for Pregnancy."

Johns Hopkins Medicine: "Blood Test."

March of Dimes: "Cystic Fibrosis and Pregnancy," "Rubella and Pregnancy."

Reviews in Obstetrics and Gynecology: "Noninvasive Prenatal Testing: The Future Is Now."

Stanford Children's Health: "Blood types in pregnancy," "First trimester screening."