Blood tests in pregnancy and after birth
For anyone who is pregnant or has given birth in the past year, and the people helping them keep track of results.
9 tests explainedAI-verified against cited sources. Awaiting expert reviewLast reviewed 2026-10-19
Pregnancy changes many blood results, and most of these changes are expected. Blood volume grows, so hemoglobin often dips. The pregnancy hormone hCG pulls TSH lower early on, and platelets often drift down late in pregnancy. Some tests are read against ranges for each trimester, and a few checks are timed to set weeks, during pregnancy and after birth.
The tests that matter most
Most important first. Each one links to its full page, where the note for this group sits near the top.
Hemoglobin
Hemoglobin falls in the first and second trimesters because blood volume expands, so pregnancy has its own anemia cutoffs. The USPSTF notes that ACOG, the AAFP and the CDC recommend checking for anemia at the first prenatal visit, and that ACOG recommends checking again at 24 to 28 weeks.
Sources: Centers for Disease Control and Prevention (1998) and U.S. Preventive Services Task Force (2024)
- Men, 16+
- 13–17.7 g/dL
- Women, 16+
- 11.1–15.9 g/dL
Ferritin
Iron needs rise in pregnancy, and the CDC notes that iron stores are hard to keep up in the second and third trimesters without iron supplements. The USPSTF notes there is no agreed ferritin level that marks iron deficiency, and the CDC notes it is unclear whether a normal result early in pregnancy predicts iron status later on.
Sources: U.S. Preventive Services Task Force (2024) and Centers for Disease Control and Prevention (1998)
- Men, 20–59
- 30–400 ng/mL
- Men, 60+
- 31–409 ng/mL
TSH (Thyroid Stimulating Hormone)
TSH is lowest near the end of the first trimester, because hCG, a pregnancy hormone, acts on the thyroid's TSH receptor. The ATA's 2026 guideline prefers the lab's own trimester-specific ranges and, when none are available, says a range of 0.1 to 4.0 mIU/L can be used in the first and second trimesters.
Source: Thyroid (2026)
- Usual adult range
- 0.45–4.5 mIU/L
Free T4 (Free Thyroxine)
Free T4 is read together with TSH. The ATA's 2026 guideline prefers the lab's own trimester-specific free T4 ranges, notes that free T4 results differ a lot between test methods, and says TSH usually reflects thyroid function best in pregnancy.
Source: Thyroid (2026)
- Usual adult range
- 0.82–1.77 ng/dL
Glucose (1-Hour, 50 g Challenge Screen)
The ADA suggests screening for gestational diabetes at 24 to 28 weeks. In the two-step approach, this 1-hour glucose drink test comes first, and a high result leads to a longer glucose tolerance test.
Sources: American Diabetes Association (2026) and National Institute of Diabetes and Digestive and Kidney Diseases
Glucose, OGTT 2-Hour
The glucose tolerance test is used to check for gestational diabetes. After gestational diabetes, the ADA suggests this test again 4 to 12 weeks after birth, then lifelong checks for prediabetes or diabetes every 1 to 3 years.
- Usual adult range
- up to 139 mg/dL
Platelet Count
Platelet counts often drift lower in pregnancy, most often in the third trimester. This mild dip, called gestational thrombocytopenia, is common, is not linked to harm for parent or baby, and usually recovers after birth. Clinicians watch how low it goes, since less common causes such as preeclampsia can also lower platelets.
Source: Cureus (2022)
- Usual adult range
- 150–450 ×10³/µL
Thyroid Peroxidase Antibodies (TPO)
TPO antibodies are a sign of thyroid autoimmunity. The ATA says people who have them have a much higher risk of postpartum thyroiditis, a thyroid inflammation in the year after birth that is often short-lived, and its 2026 guideline notes the result can help with counseling about that risk.
Sources: American Thyroid Association and Thyroid (2026)
- Usual adult range
- 0–34 IU/mL
Hemoglobin A1c (HbA1c)
Before 15 weeks, the ADA suggests testing people with risk factors for diabetes that may have been there before pregnancy, and A1c is one of the tests used. The ADA also lists pregnancy among conditions that change how A1c relates to blood glucose, and suggests glucose-based criteria in those conditions.
- Usual adult range
- 4.8–5.6 %
Ranges shown are the usual adult ranges labs print. The range on your own report is the one your result is judged against.
Getting results you can compare
- Note the week of pregnancy with each result. Hemoglobin and thyroid tests are read against cutoffs or ranges for each trimester, so the same number can mean different things at different points.
Sources: Centers for Disease Control and Prevention (1998) and Thyroid (2026)
- Thyroid test methods differ, so the ATA's 2026 guideline suggests using the same method for follow-up tests throughout a pregnancy. Noting the lab with each result makes changes easier to compare.
Source: Thyroid (2026)
- After birth, the CDC suggests a hemoglobin or hematocrit check at 4 to 6 weeks for people at risk of anemia, such as after heavy blood loss during delivery, anemia that lasted into the third trimester, or a multiple birth.
- After postpartum thyroiditis, once thyroid results are back to normal, the ATA's 2026 guideline suggests repeating thyroid tests after one year, or sooner if signs of an underactive thyroid appear.
Source: Thyroid (2026)
Good to know
- A TSH below the usual adult range is common near the end of the first trimester. The ATA's 2026 guideline explains that hCG gently stimulates the thyroid at this stage, and that this short-lived pattern is not linked to pregnancy problems. Clinicians tell it apart from less common causes such as Graves' disease.
Source: Thyroid (2026)
- Thyroid levels can swing in the year after birth. The ATA says postpartum thyroiditis affects about 5 to 10% of women in the U.S. An overactive phase often starts 1 to 4 months after delivery, an underactive phase can follow at 4 to 8 months, and most people's thyroid works normally again within 12 to 18 months after symptoms start.
Source: American Thyroid Association
- Advice on iron testing differs. The USPSTF notes that ACOG, the AAFP and the CDC recommend anemia screening in pregnancy. The USPSTF itself found too little evidence to weigh the benefits and harms of routine screening for iron deficiency, or of routine iron supplements, in pregnant people without symptoms. Your clinician can explain which approach they follow.
Questions you could ask
- Which trimester ranges are you using for my thyroid results?
- When will my glucose screening be, and which test will it be?
- Should we recheck my hemoglobin or ferritin later in pregnancy?
- If my platelets are lower, how often will we recheck them?
- Should my thyroid be checked in the months after birth?
Sources
- Recommendations to Prevent and Control Iron Deficiency in the United States (MMWR Recommendations and Reports), Centers for Disease Control and Prevention (1998)
- Iron Deficiency and Iron Deficiency Anemia During Pregnancy: Screening and Supplementation (Final Recommendation Statement), U.S. Preventive Services Task Force (2024)
- American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum, Thyroid (2026)
- Postpartum Thyroiditis (patient information), American Thyroid Association
- 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026, American Diabetes Association (2026)
- Gestational Diabetes, National Institute of Diabetes and Digestive and Kidney Diseases
- Gestational Thrombocytopenia: A Review on Recent Updates, Cureus (2022)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-19. How we build and check these pages.
Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.