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Your first trimester

Weeks 0–13 (through 13 weeks 6 days)

The first trimester runs from the first day of your last period through 13 weeks 6 days. In these weeks the brain, spinal cord, heart and other major organs begin to form, and by the end the fetus has fingers and toes. Many people notice early symptoms such as nausea and tiredness, though every pregnancy is different.

Gouache illustration of a germinating seed with its first two leaves
First0 to 13+6
Second14+0 to 27+6
Third28+0 to birth
Weeks of pregnancy counted from the first day of the last period, using ACOG trimester boundaries. The apricot band marks early, full and late term (37+0 to 41+6).

How your baby develops

  • Pregnancy is counted from the first day of your last period, so in weeks 1 and 2 you are not yet pregnant. Conception usually happens around week 3, and the early embryo implants in the lining of the uterus around week 4.1,2
  • Around week 5, the brain, spinal cord and heart begin to develop. By about weeks 6 to 7, the heart beats with a regular rhythm that can be seen on a vaginal ultrasound.2
  • The eyes and ears begin to form around weeks 6 to 7, and the lungs start to form around week 8.2,3
  • By about weeks 8 to 9, all of the essential organs have begun to grow. They keep developing for the rest of pregnancy.2,3
  • Between about 9 and 12 weeks, fingers, toes and nails form, and hair follicles start to develop.2,4
  • Toward the end of the trimester the genitals appear, the eyelids close, and the fetus can open and close its hands. It is still too early to feel any movement.2,3
  • From around 9 to 10 weeks, your provider may be able to hear the heartbeat with a handheld Doppler device.2,3

Changes you may notice

  • Nausea, with or without vomiting, is common in early pregnancy. Despite the name morning sickness, it can happen at any time of day, and not everyone has it.4
  • Many people have sore breasts and feel more tired than usual in the first weeks.4
  • You may need to pee more often, and mood swings are common.4
  • Some people notice acne or other skin changes, or mild shortness of breath. Every person and every pregnancy is different.4
  • If you are being sick often and cannot keep food down, tell your midwife or doctor as soon as possible, because there is a risk of becoming dehydrated.5

Care in this trimester

  • In the US, people often have two or three prenatal visits in the first trimester. The first visit typically includes working out your due date, checking weight and blood pressure, urine tests, and blood tests such as Rh factor and iron levels. A pelvic exam, a Pap test if you are due for one, and STI tests may also be done. In the UK, the booking appointment is usually between 8 and 12 weeks.4,6
  • An ultrasound in the first trimester, up to 13 weeks 6 days, is the most accurate way to confirm or set your due date. In the UK, the dating scan is offered between 11 weeks 2 days and 14 weeks 1 day.6,7,8
  • Screening for chromosomal conditions should be offered to everyone. Cell-free DNA screening can be done from 10 weeks, and combined first-trimester screening is done between 10 weeks 0 days and 13 weeks 6 days.9
  • Your care team will typically talk about what to avoid during pregnancy, such as alcohol, tobacco, recreational drugs, hot tubs and saunas, and some foods, including raw or undercooked meat and fish, high-mercury fish and unpasteurized products.4

Questions to ask your provider

  • Based on my dates and any ultrasound, what is my due date?
  • Which screening tests for chromosomal conditions are available to me, and when would they be done?
  • Are any of my current medicines or supplements something I should review?
  • What can I do if nausea is making it hard to eat or drink?
  • Who should I contact if I have bleeding or pain before my next appointment?

Week by week

When to contact your provider straight away

Call 911 (US) or 999 (UK) in an emergency, such as heavy bleeding or bleeding with severe pain, a seizure, chest pain, or severe trouble breathing.12,13,14,15

These symptoms are not always serious, but each one needs prompt advice from your provider, midwife or maternity unit (or NHS 111 in the UK if you cannot reach them) rather than waiting for your next appointment, and it helps to tell them you are pregnant.

  • Vaginal bleeding or spotting at any stage of pregnancy. Call 911 or 999 if the bleeding is heavy or comes with severe belly pain, shoulder pain, or feeling faint or dizzy.14,16
  • Fluid leaking or gushing from your vagina, or discharge that is unusual for you. Get checked straight away if the fluid is greenish, colored or smells bad.14,15,17,18
  • Your baby moving less than usual, not moving, or a change in their usual pattern of movements. Call immediately, even in the middle of the night, and do not use a home Doppler to check yourself.14,19
  • A severe headache that does not go away with simple painkillers or gets worse, or sudden changes in your vision such as blurring or seeing flashing lights.12,14,15
  • Sudden swelling of your face, hands or feet, or swelling, redness or pain in a leg.12,14,15
  • Belly pain that is severe or does not go away after resting, or pain just below the ribs.12,14,20
  • Regular tightenings or contractions, period-type pains, or backache that is unusual for you before 37 weeks, which can be signs of preterm labor.18,20
  • A fever of 100.4°F (38°C) or higher, or feeling hot, cold or shivery.14,15,18
  • Shortness of breath that comes on suddenly or happens when you lie flat, chest pain, a racing or pounding heart, or dizziness or fainting.14,15
  • Very itchy skin, especially on the palms of your hands or soles of your feet, or itching that is worse at night.21
  • Being sick so often that you cannot keep food or drink down, or signs of dehydration such as dark urine, peeing very little, or feeling dizzy.5,14
  • Thoughts of harming yourself or your baby. Get help right away, and call 911 or 999 if you think you might act on them.14,22

Questions about the first trimester

When does the first trimester end?

Using ACOG's definition, the first trimester lasts through 13 weeks 6 days, and the second trimester starts at 14 weeks 0 days. Some resources, including the NHS week-by-week guide, group week 13 with the second trimester, so you may see slightly different cutoffs.1,10

Why am I counted as pregnant before I conceived?

Pregnancy is dated from the first day of your last menstrual period, and the due date is 280 days after that date. Conception usually happens about two weeks later, so in weeks 1 and 2 you are not actually pregnant yet.1,7

Will morning sickness stop after the first trimester?

For many people, nausea and tiredness ease in the second trimester. Some people continue to feel sick well into the second trimester.11

When is the first ultrasound?

In the US, most people have at least one ultrasound in the first trimester. In the UK, the dating scan is usually between 11 and 14 weeks.4,6

Sources

  1. How Your Fetus Grows During PregnancyAmerican College of Obstetricians and Gynecologists
  2. Fetal developmentUS National Library of Medicine
  3. Fetal Development: Week-by-Week Stages of PregnancyCleveland Clinic
  4. First Trimester of Pregnancy: Weeks & What To ExpectCleveland Clinic
  5. Severe vomiting in pregnancyNHS (England)
  6. Your antenatal appointmentsNHS (England)
  7. Committee Opinion No. 700: Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists, with AIUM and SMFM, 2017
  8. Antenatal care (NG201)National Institute for Health and Care Excellence, 2021
  9. Practice Bulletin No. 226: Screening for Fetal Chromosomal AbnormalitiesAmerican College of Obstetricians and Gynecologists, 2020
  10. Week-by-week guide to pregnancyNHS Best Start in Life
  11. Second Trimester of Pregnancy: What To ExpectCleveland Clinic
  12. Pre-eclampsiaNHS (England)
  13. Epilepsy and pregnancyNHS (England)
  14. Urgent Maternal Warning Signs and Symptoms (Hear Her campaign)US Centers for Disease Control and Prevention
  15. Pregnancy symptoms you need to get help forNHS (England)
  16. Vaginal bleeding in pregnancyNHS (England)
  17. Signs of going into labourNHS Best Start in Life
  18. Premature labour and birthNHS (England)
  19. Your baby's movementsNHS (England)
  20. Stomach pain in pregnancyNHS (England)
  21. Itching in pregnancyNHS (England)
  22. Depression in pregnancyNHS (England)

Links checked November 12, 2026. Citing a source does not mean it endorses this site.