Free family guide · Pregnancy week by week

First trimester: weeks, symptoms and tests

The Office on Women's Health (OWH) counts the first trimester as weeks 1 to 12.[1] The NHS week-by-week guide counts it as weeks 4 to 12, and ACOG counts it from the first day of the last period to 13 weeks 6 days.[2][3]

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Sources on this page: OWH; NHS; ACOG; MedlinePlus; NICHD; WHO; USPSTF; CDC; NIMH; HRSA

This page shares what health organizations say. It is not medical advice. It is general information. Talk to your doctor, midwife or pediatrician about your own situation.

In an emergency, call 911.

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Weeks in this trimester

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Your body

OWH lists these common changes in the first trimester:[1]

  • tiredness;[1]
  • tender breasts;[1]
  • nausea, with or without vomiting.[1]

MedlinePlus lists other common symptoms of pregnancy:[4]

  • fatigue;[4]
  • needing to urinate often, and some leaking;[4]
  • vaginal discharge;[4]
  • constipation and heartburn;[4]
  • nosebleeds and bleeding gums.[4]

MedlinePlus says hyperemesis gravidarum is extreme, lasting nausea and vomiting that can cause dehydration and weight loss.[5] It says people with milder morning sickness can usually still eat and drink some of the time.[5]

Sources differ

Sources describe how long morning sickness lasts in different ways:[6][7]

  • ACOG says it usually starts before 9 weeks, and for most women it goes away by 14 weeks.[6] It says for some it lasts several weeks or months, and for a few the whole pregnancy.[6]
  • The NHS says it usually clears up by weeks 16 to 20.[7]

Your baby

  • OWH says the brain and spinal cord begin to form in the first trimester.[1]
  • OWH says the heart beats steadily by about week 8.[1]
  • OWH says that by week 12 the baby is about 3 inches long and can make a fist.[1]

Visits and tests

NICHD says the first prenatal visit usually happens after about week 8.[8] The World Health Organization (WHO) recommends a first contact within the first 12 weeks.[9]

The American College of Obstetricians and Gynecologists (ACOG) says before 10 weeks after your last period is the ideal time to start prenatal care.[10]

OWH lists these tests in the first trimester:[11]

  • initial blood tests at the first visit;[11]
  • chorionic villus sampling (CVS) at weeks 10 to 13;[11]
  • the first trimester screen at weeks 11 to 14.[11]

MedlinePlus says a first-trimester ultrasound, often done through the vagina, helps estimate the due date.[12]

ACOG says first-trimester screening, a blood test plus an ultrasound measurement called nuchal translucency, is usually done between 10 and 13 weeks.[13] It says cell-free DNA screening can be done from 10 weeks.[13]

MedlinePlus says prenatal testing is optional and is your decision, made with your provider.[14]

Taking care of yourself

  • The US Preventive Services Task Force advises a daily folic acid supplement through the first 2 to 3 months of pregnancy.[15]
  • MedlinePlus says most people need no extra calories in the first trimester.[16]
  • CDC suggests at least 150 minutes a week of moderate activity, such as 30 minutes on 5 days, unless there is a medical reason not to.[17]
  • CDC says to avoid lying flat on your back during exercise after the first trimester.[17]
  • MedlinePlus says pregnant people should keep caffeine under 200 mg a day.[18]
  • CDC says there is no known safe amount of alcohol during pregnancy.[19]
  • CDC's Yellow Book says pregnancy complications are most likely in the first and third trimesters.[20]

Feelings check-in

The National Institute of Mental Health (NIMH) says perinatal depression includes depression during pregnancy.[21] It says signs last at least 2 weeks and can include sadness or emptiness, loss of interest, guilt, and changes in sleep and appetite.[21]

HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[22]

When to call or get help

CDC lists vaginal bleeding or leaking fluid, severe belly pain that does not go away, and severe nausea and vomiting among its urgent warning signs in pregnancy.[23]

MedlinePlus says any bleeding in pregnancy should prompt a call to a provider.[24]

MedlinePlus says early signs of an ectopic pregnancy may include light bleeding and pelvic pain, and that it can be a medical emergency.[25]

Questions families ask

How many weeks is the first trimester?

OWH counts it as weeks 1 to 12.[1] The NHS guide counts it as weeks 4 to 12, and ACOG counts it to 13 weeks 6 days.[2][3]

What are common first trimester symptoms?

OWH lists tiredness, tender breasts and nausea, with or without vomiting.[1]

What tests are offered in the first trimester?

OWH lists initial blood tests, CVS at weeks 10 to 13 and the first trimester screen at weeks 11 to 14.[11] MedlinePlus says prenatal testing is optional.[14]

Do I need extra calories in the first trimester?

MedlinePlus says most people need no extra calories in the first trimester.[16]

How long does morning sickness last?

ACOG says for most women it goes away by 14 weeks.[6] The NHS says it usually clears up by weeks 16 to 20.[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.

  1. Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
  2. NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
  3. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
  4. MedlinePlus (National Library of Medicine). Common symptoms during pregnancy. Checked October 24, 2026.
  5. MedlinePlus (National Library of Medicine). Hyperemesis gravidarum. Checked October 24, 2026.
  6. American College of Obstetricians and Gynecologists. Morning Sickness: Nausea and Vomiting of Pregnancy. Checked October 24, 2026.
  7. NHS (United Kingdom). Vomiting and morning sickness. Checked October 24, 2026.
  8. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
  9. World Health Organization. New guidelines on antenatal care for a positive pregnancy experience (news release, Nov 7, 2016). Checked October 24, 2026.
  10. American College of Obstetricians and Gynecologists. Prenatal Care. Checked October 24, 2026.
  11. Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
  12. MedlinePlus (National Library of Medicine). Prenatal care in your first trimester. Checked October 24, 2026.
  13. American College of Obstetricians and Gynecologists. Prenatal Genetic Screening Tests. Checked October 24, 2026.
  14. MedlinePlus (National Library of Medicine). Prenatal Testing. Checked October 24, 2026.
  15. US Preventive Services Task Force. Folic Acid Supplementation to Prevent Neural Tube Defects: Preventive Medication. Checked October 24, 2026.
  16. MedlinePlus (National Library of Medicine). Pregnancy and Nutrition. Checked October 24, 2026.
  17. Centers for Disease Control and Prevention. Pregnant & Postpartum Activity: An Overview. Checked October 24, 2026.
  18. MedlinePlus (National Library of Medicine). Caffeine in the diet. Checked October 24, 2026.
  19. Centers for Disease Control and Prevention. About Alcohol Use During Pregnancy. Checked October 24, 2026.
  20. Centers for Disease Control and Prevention (CDC Yellow Book). Pregnant Travelers. Checked October 24, 2026.
  21. National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
  22. Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
  23. Centers for Disease Control and Prevention (HEAR HER campaign). Urgent Maternal Warning Signs and Symptoms. Checked October 24, 2026.
  24. MedlinePlus (National Library of Medicine). Miscarriage. Checked October 24, 2026.
  25. MedlinePlus (National Library of Medicine). Ectopic Pregnancy. Checked October 24, 2026.

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