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Tests During Pregnancy: Prenatal Screenings in the First, Second and Third Trimester

Pregnant patient discusses upcoming prenatal screenings with her doctor
A pregnant patient smiling and holing her stomach during a physical exam with a YNHH OBGYN physician.

Pregnant patients can expect to have many different appointments in the weeks leading up to their baby’s birth. While each patient has their own unique needs, there are some screenings that everyone will receive.

A full-term pregnancy is 40 weeks long and is broken down into three trimesters. The first trimester lasts from 0 to 13 6/7 weeks gestation, the second trimester lasts from 14 to 28 weeks and the third trimester lasts from 29 weeks on.

First trimester screenings

Once someone finds out they’re pregnant, their provider will want to confirm the pregnancy with a blood or urine test, which is typically followed by an initial ultrasound.

Next, patients can expect plenty of bloodwork. This will confirm the patient’s blood type and whether they have antibodies to red blood cell proteins. Blood work will also be done to check for active hepatitis B, hepatitis C, HIV, gonorrhea, chlamydia, and syphilis. If any of those are present, it will be important to initiate treatment.

Early on, a provider will also conduct a pap smear if the patient is overdue.

“We use this opportunity to perform routine healthcare maintenance,” said high risk pregnancy specialist Katherine Kohari, MD, associate director of Yale New Haven Children’s Hospital’s Fetal Care Center and assistant professor of Obstetrics, Gynecology and Reproductive Sciences at Yale School of Medicine. “A lot of patients in their reproductive years only seek out medical care in pregnancy, so we really use that opportunity to affect large scale preventative health measures.”

Genetic testing during pregnancy

The first trimester is also a time when providers order genetic testing. An initial blood test can determine a patient’s genetic status when it comes to carrying mutations. For example, this test can help evaluate the probability of the pregnant person carrying a mutation in a gene that may increase the risk for their baby to have a condition such as cystic fibrosis, sickle cell disease, spinal muscular atrophy, or fragile X syndrome.

Noninvasive prenatal testing (NIPT) is another blood test that will determine the sex of the baby and looks at genetic chromosomal abnormalities with the pregnancy, namely Trisomy 21 or Down Syndrome. If that test comes back abnormal, or if the patient wants a diagnostic test, there are a few options.

One is called chorionic villus sampling (CVS). During this test, which is typically done between 10 and 14 weeks, a provider will insert a needle through the abdomen or through the cervix into the placenta, to collect a small sample. The placental tissue will then undergo genetic testing.

The other option is an amniocentesis, which is typically performed after 16 weeks. During that test, the sample is coming from the amniotic fluid around the fetus.

Second trimester screenings

Once patients hit the second trimester, there are a couple of screenings they may be nervous about. One is the glucose challenge test (GCT) which checks for gestational diabetes. During this test, patients drink a sugary drink and then have their blood sugar measured an hour later. Patients who fail the GCT will be asked to complete a fasting 3-hour glucose tolerance test.

Patients diagnosed with gestational diabetes will need to adjust their diet and may need medication to help safely manage their gestational diabetes throughout the rest of their pregnancy.

Anatomy scan

Another key screening is the anatomy scan, which is typically done between 18 and 22 weeks. This can be a lengthy appointment, lasting from 45 minutes to an hour. During the visit, a patient will undergo a detailed ultrasound, with the sonographer measuring and taking photos of many different parts of the baby.

“We look at the placenta, we look at the umbilical cord, we look at the cervix,” said Dr. Kohari. “We measure the size of the fetal head, the length of the bones, all of the main organ systems, so there’s a variety of things we look at there.”

Additionally, some patients require a fetal echocardiogram, which can help detect developmental abnormalities of the heart. This ultrasound is typically performed around 22 weeks of pregnancy. Patients with a personal or family history of a congenital heart defect, those who conceived through IVF, or those taking certain medications may be recommended to complete a fetal echocardiogram.

Third trimester screenings

Once patients reach the third trimester, they undergo more bloodwork that will check for issues with platelets, anemia and a repeat check for syphilis and HIV.

Around 32 weeks, some patients may require a nonstress test. During this test, a fetal heart rate monitor and a contraction monitor are placed on the abdomen and then the baby’s heart rate and uterine activity is monitored for about 30 minutes. Nonstress tests are recommended for any pregnancy where a provider needs to keep a closer eye on their patient. Some scenarios include:

  • A patient with chronic or gestational hypertension 
  • Increased risk for a complication such as stillbirth: often preexisting maternal medical conditions
  • The baby is measuring small for gestational age
  • A patient is past their due date

Of course, in the final weeks of pregnancy, a patient will be discussing their plans for delivering their baby with their clinician.

A Group B streptococcus screen is performed, which tests for the presence of this bacteria in the vagina and rectum. If present, the patient will require antibiotics in labor. Perhaps a patient will need to undergo a planned cesarian section or be induced early. No matter the circumstances, their clinician can guide them and explain what to expect in the days or weeks to come.