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Pregnancy Week by Week: What Happens in Each Trimester (Simple Guide)
Week 1 of pregnancy starts on the first day of your last period, before you are pregnant at all. MedlinePlus counts it that way, and so does every clinic, which is why "40 weeks" is really about 38 weeks of a baby growing. By week 8 all the major organs have begun to form; at week 20 you may feel the first flutter; at 39 weeks the Office on Women's Health calls your baby full term. Everything else is detail. Not sure of your week? Start with the due-date guide.
How the trimesters are counted
The U.S. Office on Women's Health divides pregnancy into weeks 1–12, 13–28 and 29–40. You will see other splits online (1–13, 14–27, 28–40 is common), and neither is wrong; the borders are conventions, not biology. This article uses the Office on Women's Health split, and the development notes below come from its stages-of-pregnancy page and MedlinePlus's fetal development article.
First trimester (weeks 1–12): everything happens out of sight
Nothing shows. Almost everything is being built.
- Weeks 3–4: conception, then the blastocyst attaches to the wall of the uterus and starts drawing nourishment from your blood.
- Week 5: the brain, spinal cord and heart begin to develop.
- Weeks 6–7: the heart beats with a regular rhythm; eyes and ears begin to form; limb buds grow.
- Week 8: all major organs and external body structures have begun to form. Hands and feet look like little paddles. Length is nearly 1 inch (about 2.5 cm), weight under one-eighth of an ounce.
- Week 10: MedlinePlus marks the end of week 10 as the point where the embryo becomes a fetus.
- Week 12: nerves and muscles start working together and the baby can make a fist; the external sex organs show. About 3 inches (7.5 cm) and almost an ounce.
For you, the Office on Women's Health lists extreme fatigue, tender breasts, nausea with or without vomiting, food cravings, mood swings, constipation, frequent urination, headaches and heartburn. Not all of them arrive, and none of them are graded. If the nausea is severe, read our morning sickness guide.
Your checklist: book the first prenatal visit as soon as the test is positive (MedlinePlus and NICHD both say soon after you learn you are pregnant), and start folic acid if you have not. NICHD puts the dose at 400 micrograms a day and the payoff at a 70% lower risk of neural tube defects. An optional first-trimester screen is offered at 11–14 weeks.
Second trimester (weeks 13–28): the part people enjoy
- Week 16: muscle and bone keep forming, skin begins to form, and the baby makes sucking motions. About 4 to 5 inches (10–13 cm) and almost 3 ounces (85 g).
- Weeks 19–21: quickening. MedlinePlus places the first movements a mother feels here; the Office on Women's Health describes "slight fluttering" at week 20. The baby can hear now, and is covered in fine hair (lanugo) and a waxy coating (vernix). About 6 inches (15 cm) and 9 ounces (255 g) at week 20.
- Week 22: eyebrows and lashes appear, and the heartbeat can be heard with a stethoscope.
- Week 24: footprints and fingerprints have formed, taste buds appear, bone marrow starts making blood cells. The lungs are formed but do not yet work. About 12 inches (30 cm) and 1½ pounds (680 g).
- Week 26: all parts of the eyes are developed.
The Office on Women's Health's second-trimester list is mostly skin and joints: stretch marks, a dark line from navel to pubic bone (linea nigra), darker patches on the face, numb or tingling hands (carpal tunnel), itching and some swelling. Most of it fades after birth.
Your checklist: the standard anatomy ultrasound falls at 18–20 weeks, and the glucose screening test at 24–28 weeks (the CDC's window; the Office on Women's Health lists 26–28). Both are on the prenatal appointment schedule.
Janeen shows exactly which week you're in — baby's fruit-size, length and weight, what's happening in your body, plus a kick counter, symptom diary and smart checkup reminders matched to your week. Free on iOS.
Download Janeen freeThird trimester (weeks 29–40): growing, then arriving
- From week 28: the Office on Women's Health says to keep track of your baby's movement every day so you know what is normal for you. Our kick counter guide explains the count-to-10 method and when to call.
- Weeks 27–30: the brain grows rapidly and the eyelids can open and close.
- Week 32: bones are fully formed but still soft; the eyes sense changes in light; practice "breathing" movements begin; the body starts storing iron and calcium. About 15 to 17 inches (38–43 cm) and 4 to 4½ pounds (1.8–2 kg).
- Week 36: vernix thickens and body fat keeps building. About 16 to 19 inches (41–48 cm) and 6 to 6½ pounds (2.7–2.9 kg).
- Weeks 37–40: at 39 weeks the baby is considered full term, with organs ready to work on their own. Expect 6 pounds 2 ounces to 9 pounds 2 ounces (2.8–4.1 kg) and 19 to 21 inches (48–53 cm) at birth.
Your body: shortness of breath, heartburn, swelling, hemorrhoids, colostrum leaking from the breasts, a belly button that pops out, trouble sleeping, the baby "dropping" lower, and contractions. That last one deserves its own page: how to tell practice contractions from labor.
Your checklist: visits move to every two weeks from 28 weeks and weekly from 36, and the group B strep swab is done at 36–37 weeks. Pack the hospital bag before week 36 rather than after; the last weeks go faster than they look on the calendar.
Baby size by week
The lengths and weights are the Office on Women's Health figures, with rounded metric equivalents. The fruit is ours; it is a memory aid, not a measurement.
| Week | About the size of | Length / weight |
|---|---|---|
| 8 | Raspberry 🫐 | Nearly 1 in (2.5 cm) / under ⅛ oz (3 g) |
| 12 | Lime 🍋 | About 3 in (7.5 cm) / almost 1 oz (28 g) |
| 16 | Avocado 🥑 | 4–5 in (10–13 cm) / almost 3 oz (85 g) |
| 20 | Banana 🍌 | About 6 in (15 cm) / about 9 oz (255 g) |
| 24 | Corn cob 🌽 | About 12 in (30 cm) / about 1½ lb (680 g) |
| 32 | Squash 🎃 | 15–17 in (38–43 cm) / 4–4½ lb (1.8–2 kg) |
| 36 | Papaya 🍈 | 16–19 in (41–48 cm) / 6–6½ lb (2.7–2.9 kg) |
| 39–40 | Watermelon 🍉 | 19–21 in (48–53 cm) / 6 lb 2 oz–9 lb 2 oz (2.8–4.1 kg) |
Week-by-week FAQ
How many weeks is each trimester?
The U.S. Office on Women's Health counts the first trimester as weeks 1–12, the second as weeks 13–28 and the third as weeks 29–40. Some clinics draw the lines a week earlier or later.
When can you feel the baby move?
MedlinePlus places quickening, the first movements a mother feels, at about weeks 19 to 21, and the Office on Women's Health describes slight fluttering around week 20. After 28 weeks, track movement daily and call your provider if it drops below your normal.
When is a baby full term?
At 39 weeks, according to the Office on Women's Health, when the baby's organs are ready to function on their own. Babies born at term typically weigh between 6 pounds 2 ounces and 9 pounds 2 ounces.
When do I stop counting in weeks and start months?
Never, if you ask a clinic; pregnancy care runs on weeks. When relatives ask which month you are in, divide the week by 4.35 and round.
The bottom line
Pregnancy makes more sense as a sequence of weeks than as a fog of nine months: organs by week 8, fluttering around 20, daily kick counts from 28, full term at 39. You do not need to memorize any of it. Janeen tracks your exact week automatically and tells you what matters right now, from the first prenatal visit to the hospital bag. Near term, read our guides to signs of labor and pregnancy symptoms by stage.
Official sources
- HHS Office on Women's Health: Stages of pregnancy — accessed September 8, 2026.
- HHS Office on Women's Health: Prenatal care and tests — accessed September 8, 2026.
- MedlinePlus: Fetal development — accessed September 8, 2026.
- MedlinePlus: Prenatal care in your first trimester — accessed September 8, 2026.
- NICHD: What is prenatal care and why is it important? — accessed September 8, 2026.
- CDC: About Gestational Diabetes — accessed September 8, 2026.