Routine Prenatal Laboratory Tests
Introduction

One of the goals of prenatal care is to ensure that both the baby and the mother are healthy throughout pregnancy.   Prenatal laboratory tests help doctors in the medical care of expectant patients.  Since there are several laboratory studies performed during pregnancy, these tests can sometimes be confusing for patients.  The following explanation of laboratory tests commonly ordered during pregnancy has been written to clarify the purpose and importance of each of these studies.


Complete blood count

This test analyzes the different cell types present in a patient’s blood.  These cell types are the immune system cells called white blood cells, the oxygen carrying cells called red blood cells and cell fragments that promote clotting called platelets.  A complete blood count is most commonly used to help assess for possible infections and anemia but may also assist in the diagnosis of certain vitamin deficiencies, bleeding disorders and some forms of cancer.  Repeat screening for anemia between twenty-four to twenty-eight weeks with a second complete blood count is often performed in conjunction with screening for pregnancy-related diabetes.  


ABO and Rh type

There are many different blood types.  This lab test identifies a patient’s specific blood type and whether they are Rh negative or positive.


Antibody screen

This test screens for antibodies in a patient’s blood that may be potentially harmful to her baby.  Antibodies are part of the immune system that help to respond to foreign material in the mother’s body.  Some antibodies can cross the placenta and accordingly may present a risk to the pregnancy by attacking the baby as foreign to the mother.  If the patient is known to be Rh negative, additional tests for the presence of Rh positive antibodies are repeated at twenty-eight weeks and again after delivery before giving the patient a Rhogam injection.

After the nurses have helped to prepare you for your surgery, an anesthesiologist or anesthetist will help to make you comfortable. This is most often done by placing an epidural or spinal in your lower back through which medications can be given. This type of anesthesia will help you avoid discomfort during surgery but allow you to remain awake so you can hear your baby’s first cry. Occasionally, a patient may require general anesthesia for her cesarean delivery which means she will be asleep during the procedure.


Rubella IgG

This lab test screens for immunity against German measles.  If the patient has not developed immunity to rubella after previously receiving an MMR vaccine series, an additional booster shot can be administered after delivery.


Screening for sexually transmitted infections

More than one million individuals in the United States are infected with HIV or have AIDs of which women is the fastest growing group of new HIV infections. Pregnancy represents an important time for screening as to a woman’s HIV status, as well as other sexually transmitted infections, since these illnesses can affect the health of both the mother and her baby if they remain untreated.  The American College of Obstetricians and Gynecologists recommends that all pregnant women be tested for HIV and other sexually transmitted infections including syphilis, chlamydia, and gonorrhea.  These tests involve a combination of blood tests and swabs of cervical secretions.


Hepatitis B surface antigen

This laboratory study screens patients for both new and chronic hepatitis B infections.


Urine culture

A urine culture is collected at the onset of prenatal care to screen for urinary tract infections.  Not all urinary tract infections will present with symptoms of burning with urination or frequent urination.  However, if a urinary tract infection is allowed to worsen during pregnancy it can potentially result in a kidney infection and lead to other pregnancy complications such as preterm labor.


Pap smear

Guidelines from the American College of Obstetricians and Gynecologists released in 2009 recommend that screening for cervical cancer using Pap smears start at the age of twenty-one. For patients with normal results, Pap smears should be repeated every three to five years, depending on the patients age and whether they have HPV testing or not. For individuals with abnormal Pap smear results, the frequency of repeat Pap smears is determined based on the severity of the abnormality involved.


Screening for cystic fibrosis

The American College of Obstetricians and Gynecologists has recommended that all couples who are considering pregnancy or are currently pregnant be screened for cystic fibrosis (CF) early during pregnancy through a simple blood test. The goal of screening for carriers of CF is to identify couples who may be at risk for having a child with CF and to provide them with information about their reproductive choices.


Quadruple screen

The quadruple screen or “quad” screen tests for the fetal genetic risks of Down Syndrome, a similar genetic condition referred to as Edward’s Syndrome, and early brain and spine developmental problems such as Spina Bifida.  It can also provide additional information about other abnormalities that can affect a baby’s health and growth that may prove helpful to a patient’s doctor in caring for her pregnancy. 


Glucose screening test

Diabetes during pregnancy is referred to as gestational diabetes.  Since diabetes may have few if any symptoms, women are encouraged to be tested for diabetes during pregnancy.  A one hour glucose screening test is generally performed between twenty-four and twenty-eight weeks gestation to test expectant patients for diabetes. If this screening test is suggestive of diabetes, an additional test called a three hour glucose tolerance test is performed to confirm the presence of gestational diabetes.  For those who may have risk factors for the development of diabetes, performing a glucose screening test earlier in pregnancy may be considered.


GBS culture

The bacterium group B streptococcus (GBS) is the most common cause of serious infections in newborns during the first week of life.  To reduce the frequency of serious newborn infections, medical experts have recommended that all expectant patients between thirty-five and thirty-seven weeks be screened for GBS through cultures of the rectum and vagina.  If a patient’s culture is positive, the Centers of Disease Control recommends that these mothers are treated with antibiotics during labor to protect their baby against infection.


Conclusion

We hope that this explanation has proven helpful in clarifying the purpose and importance of these studies.  Although additional lab tests may be ordered depending on a patient’s individual medical needs, these lab tests represent the most commonly ordered tests during pregnancy.  If you have any additional questions about specific laboratory tests, please feel free to discuss them further with your doctor.