Screening and diagnostic tests
- MedlinePlus says a screening test estimates risk, and an abnormal result means more information is needed.[1]
- It says a diagnostic test confirms whether a condition is present.[1]
- The American College of Obstetricians and Gynecologists (ACOG) says a positive screening result means a higher chance, not a diagnosis, and a negative result means a lower chance but does not rule a condition out.[2]
- ACOG says testing is a personal choice.[3]
The timeline OWH lists
Prenatal tests and timing as the Office on Women's Health (OWH) lists them:[4]
| Test | When OWH lists it |
|---|---|
| Initial blood tests (blood type and Rh, anemia, infections, rubella and chickenpox immunity) | First visit |
| Chorionic villus sampling (CVS) | Weeks 10 to 13 |
| First trimester screen | Weeks 11 to 14 |
| Amniocentesis | Weeks 14 to 20 |
| Maternal serum screen | Weeks 15 to 20 |
| Standard ultrasound | Often weeks 18 to 20 |
| Glucose challenge and tolerance tests | Weeks 26 to 28 |
| Nonstress test | After week 28 |
| Biophysical profile | Third trimester |
| Group B strep | Weeks 36 to 37 |
The timing ACOG gives
Prenatal tests and timing as ACOG describes them:[2][5][6][3][7]
| Test | When ACOG says it is usually done |
|---|---|
| First-trimester screening (blood test and nuchal translucency ultrasound) | Between 10 and 13 weeks[2] |
| Cell-free DNA screening | From 10 weeks[2] |
| Chorionic villus sampling (CVS) | Between 10 and 13 weeks[5] |
| Amniocentesis | Usually 15 to 20 weeks[5] |
| Quad screen | 15 to 22 weeks[2] |
| Standard ultrasound | Usually 18 to 22 weeks[6] |
| Glucose screening | Usually 24 to 28 weeks, earlier with risk factors[3] |
| Syphilis test | At the first visit, in the third trimester and at delivery[3] |
| Group B strep culture | 36 to 38 weeks[7] |
Sources differ
ACOG says there is no evidence that ultrasound harms a developing baby, and that it is still recommended only for medical reasons.[6]
At the first visit
NICHD says the first visit often includes a urine test and blood tests for blood type, Rh factor, blood count, hepatitis B, HIV, rubella and syphilis.[8] It says the visit may also include an ultrasound and an offer of genetic screening.[8]
MedlinePlus says a first-trimester ultrasound helps estimate the due date.[9]
What MedlinePlus adds for the second trimester
- A quad screen between weeks 15 and 22, most accurate at weeks 16 to 18.[10]
- Amniocentesis between weeks 14 and 20.[10]
- An anatomy ultrasound around week 20.[10]
- An antibody screen for people who are Rh-negative, with a medicine possibly given around week 28.[10]
- MedlinePlus also mentions cell-free DNA screening.[10]
The World Health Organization (WHO) recommends one ultrasound before 24 weeks.[11]
The glucose test
Sources differ
Sources give different timing for gestational diabetes testing:[12][8][13][3][4]
- The US Preventive Services Task Force (USPSTF) advises screening at 24 weeks or later.[12]
- NICHD says screening usually happens between 24 and 28 weeks.[8]
- CDC says testing is usually done at 24 to 28 weeks, and people at higher risk may be tested earlier.[13]
- ACOG says glucose screening is usually between 24 and 28 weeks.[3]
- OWH lists the glucose tests at weeks 26 to 28.[4]
Later in pregnancy
MedlinePlus says the group B strep swab is done a few weeks before the due date.[14] It says routine third-trimester lab tests and ultrasounds are not needed for everyone.[14]
USPSTF advises blood pressure screening throughout pregnancy.[15]
Questions families ask
Are prenatal tests required?
MedlinePlus says prenatal testing is optional and is your decision, made with your provider.[1]
What is the difference between screening and diagnostic tests?
MedlinePlus says a screening test estimates risk, and a diagnostic test confirms whether a condition is present.[1]
When is the anatomy ultrasound?
MedlinePlus says around week 20.[10] OWH lists a standard ultrasound, often at weeks 18 to 20.[4]
When is the group B strep test?
OWH lists it at weeks 36 to 37.[4] ACOG says the culture is done at 36 to 38 weeks.[7]
Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages
Updated 2026-10-24: first published.
Sources
All sources were opened and checked on the date shown.
- MedlinePlus (National Library of Medicine). Prenatal Testing. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Prenatal Genetic Screening Tests. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Routine Tests During Pregnancy. Checked October 24, 2026.
- Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Prenatal Genetic Diagnostic Tests. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Ultrasound Exams. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Group B Strep and Pregnancy. Checked October 24, 2026.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your first trimester. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your second trimester. Checked October 24, 2026.
- World Health Organization. New guidelines on antenatal care for a positive pregnancy experience (news release, Nov 7, 2016). Checked October 24, 2026.
- US Preventive Services Task Force. Gestational Diabetes: Screening. Checked October 24, 2026.
- Centers for Disease Control and Prevention. Gestational Diabetes. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your third trimester. Checked October 24, 2026.
- US Preventive Services Task Force. Hypertensive Disorders of Pregnancy: Screening. Checked October 24, 2026.