The first trimester spans weeks 1 through 13 of pregnancy and is the period of most rapid embryonic development. By the end of week 13, all major organs have formed, the heart is beating, and your baby measures about 7.4 cm (2.9 inches) from crown to rump. This guide covers what happens each month, which symptoms to expect, and the key medical appointments during the first trimester.
Key Takeaways
- The first trimester lasts from week 1 through week 13 (approximately 3 months)
- All major organs form during this period — the heart begins beating by week 6
- Morning sickness affects up to 80% of pregnant people, typically peaking around weeks 8–10
- The first prenatal appointment is usually scheduled between weeks 8 and 12
- Miscarriage risk drops significantly after a heartbeat is confirmed and drops further after week 12
- Folic acid (400–800 mcg daily) is critical during the first trimester for neural tube development
Month 1: Weeks 1–4
Most people don't know they're pregnant during month 1. Week 1 technically begins on the first day of your last menstrual period (LMP), even though conception usually happens around week 2.
What's happening:
- Week 2: Ovulation and conception occur. The fertilized egg (zygote) begins dividing as it travels down the fallopian tube.
- Week 3: The blastocyst implants into the uterine wall, triggering hCG production — the hormone that pregnancy tests detect.
- Week 4: The embryo is the size of a poppy seed. The neural tube (which becomes the brain and spinal cord) begins to form.
Common symptoms: Implantation bleeding (light spotting), mild cramping, breast tenderness, fatigue.
Month 2: Weeks 5–8
This is when pregnancy becomes real for most people. Symptoms ramp up and the first prenatal visit often happens.
What's happening:
- Week 5: The heart tube forms and begins to beat. Arm and leg buds appear.
- Week 6: The heartbeat can often be detected on ultrasound (90–110 bpm). The face is beginning to form — tiny nostril openings and eye lenses are developing.
- Week 7: The brain is growing rapidly. Hands and feet are forming. The embryo is about 1 cm long.
- Week 8: All major organs are present in basic form. The embryo is now officially called a fetus. Fingers and toes are beginning to form, though they're still webbed.
Common symptoms: Morning sickness (nausea with or without vomiting), food aversions, heightened sense of smell, frequent urination, fatigue, mood swings.
Month 3: Weeks 9–13
Symptoms often peak in the early part of month 3 and then begin to improve. The risk of miscarriage decreases significantly during this month.
What's happening:
- Week 9: The fetus can make tiny movements, though you won't feel them yet. Bones are starting to form.
- Week 10: All vital organs are functioning. The fetus is about the size of a strawberry (3 cm).
- Week 11: The fetus is moving actively — stretching, kicking, and hiccupping. Tooth buds form under the gums.
- Week 12: The fetus measures about 5.4 cm and weighs 14 grams. External genitalia are developing but may not be distinguishable on ultrasound yet.
- Week 13: The first trimester ends. The fetus is about 7.4 cm and has a recognizable human profile. The risk of miscarriage has dropped significantly.
Common symptoms: Nausea may begin to improve, visible weight gain begins, constipation, nasal congestion, round ligament pain.
Prenatal Care Timeline
| When | What |
|---|---|
| As soon as you know | Start prenatal vitamins with folic acid (400–800 mcg) |
| Weeks 8–12 | First prenatal appointment: medical history, blood work, urine test, possibly dating ultrasound |
| Week 10+ | NIPT (non-invasive prenatal testing) available if desired — screens for chromosomal conditions |
| Weeks 11–14 | NT scan (nuchal translucency) — measures fluid at the back of the baby's neck |
| Every 4 weeks | Routine prenatal visits through the second trimester |
What to Avoid in the First Trimester
- Alcohol — no known safe amount during pregnancy (ACOG, WHO)
- Raw or undercooked meat, fish, and eggs — risk of listeria, toxoplasma, salmonella
- High-mercury fish — swordfish, shark, king mackerel, tilefish
- Unpasteurized dairy — risk of listeria
- Cat litter — toxoplasmosis risk (have someone else handle it)
- Hot tubs and saunas — core temperature above 39°C (102°F) may increase neural tube defect risk
- Certain medications — always check with your provider before taking any medication
Tips for Surviving the First Trimester
- Eat small, frequent meals to manage nausea — crackers, ginger tea, and plain carbs often help
- Stay hydrated — dehydration worsens nausea and fatigue
- Rest when you can — first-trimester fatigue is intense and biological, not laziness
- Tell someone you trust — you don't have to wait until week 12 to tell close family or friends
- Start a pregnancy journal — track symptoms, appointments, and milestones with our Pregnancy Journal
- Use our Due Date Calculator to estimate your timeline
Frequently Asked Questions
When does the first trimester end?
The first trimester ends at the completion of week 13. The second trimester begins at week 14 and runs through week 27. This means the first trimester covers approximately the first three months of pregnancy.
What are the most common first trimester symptoms?
The most common symptoms during the first trimester are nausea and vomiting (morning sickness), fatigue, breast tenderness, frequent urination, food aversions, and mood swings. Morning sickness affects up to 80% of pregnant people and typically peaks around weeks 8–10 before improving in the second trimester.
When should I schedule my first prenatal appointment?
Most providers schedule the first prenatal appointment between weeks 8 and 12. If you have a history of complications, ectopic pregnancy, or are experiencing bleeding or severe pain, call your provider as soon as you get a positive pregnancy test for earlier evaluation.
Is it safe to exercise during the first trimester?
Yes, for most people, moderate exercise is safe and encouraged during the first trimester. Walking, swimming, prenatal yoga, and light strength training are generally recommended. Avoid contact sports, activities with a high fall risk, and exercise in extreme heat. Always discuss your exercise plan with your provider.
How common is miscarriage in the first trimester?
About 10–15% of known pregnancies end in miscarriage, with the majority occurring in the first trimester. The risk drops significantly after a heartbeat is detected (usually around week 6–7) and drops further after week 12. Most first-trimester miscarriages are caused by chromosomal abnormalities that are not preventable.