How to track your pregnancy week by week

Pregnancy is counted in weeks, not months — 40 weeks total from last menstrual period (LMP) to due date. That’s roughly 9 calendar months but actually 10 lunar months, which is why the timing feels inconsistent. This guide walks through how pregnancy is dated, what happens at each trimester, the appointments that matter, and the normal-range symptoms people often worry about unnecessarily.

How pregnancy is dated

Week 0 is the first day of your last period.Conception happens around week 2 (ovulation). This means you aren’t actually pregnant during “weeks 1 and 2” — the counting starts from LMP because most people can’t pinpoint conception day.

Due date (EDD) = LMP + 280 days. Naegele’s rule: LMP + 1 year, − 3 months, + 7 days.

If cycles are irregular or LMP is unknown, an early ultrasound (weeks 7-12) dates the pregnancy more accurately by measuring crown-rump length.

Only ~5% of babies arrive on the exact due date. “Full term” is 37-42 weeks; anything in that range is normal timing.

The three trimesters

First trimester (weeks 1-13): fertilization, implantation, organogenesis. Highest miscarriage risk (~10-15% clinically recognized; higher before pregnancy is detected). Key events: missed period, positive test, first prenatal visit, heartbeat on ultrasound around week 6-7.

Second trimester (weeks 14-27): “the good trimester” for most. Energy returns, nausea subsides, baby bump appears. Key events: anatomy scan at 18-22 weeks, feeling first movements (quickening) around 16-25 weeks, 24-week viability milestone.

Third trimester (weeks 28-40): weight gain accelerates, discomfort increases. Key events: glucose screen at 24-28 weeks, GBS test at 35-37 weeks, weekly appointments from week 36, delivery.

Appointment schedule — typical US/UK

First visit (8-10 weeks): confirms pregnancy, establishes dating, bloodwork, screening discussion.

Weeks 12-13: first-trimester screening (nuchal translucency ultrasound + blood markers) or NIPT (cell- free DNA test) for chromosomal conditions.

Weeks 16-20: check-up; possibly quad screen if not done NIPT.

Weeks 18-22: anatomy scan. Detailed ultrasound checking all fetal structures. Often when sex is confirmed if you want to know.

Weeks 24-28: glucose challenge test (screening for gestational diabetes).

Weeks 28: Rh immunoglobulin (RhoGAM) if Rh-negative.

Weeks 35-37: Group B Strep (GBS) swab; cervical check begins if relevant.

Weeks 36-40: weekly visits. Position check, fetal heart rate, cervical checks.

Week 41+: discussion of induction. Most providers induce by 41-42 weeks due to post-term risks.

What’s happening week by week — condensed

Weeks 4-8: heart starts beating (around week 6), limb buds form, neural tube closes. Most critical window for avoiding alcohol, specific medications, and environmental teratogens.

Weeks 9-13: recognizable human form, all major organs in place (though immature), reflexes begin. End of first trimester, risk of miscarriage drops substantially.

Weeks 14-20: rapid growth. External genitals differentiate (visible on ultrasound ~16-20 weeks). Movement begins; mother feels it around 18-22 weeks (earlier if a second pregnancy).

Weeks 21-27: weight increases dramatically, lung maturation begins. 24 weeks = traditional “viability threshold” (60-70% survival with NICU support; improves week by week).

Weeks 28-32: baby quadruples in weight; periods of sleep and wake. Brain development accelerates.

Weeks 33-37: baby typically moves to head-down position. Fat accumulates. Lungs approach mature function by week 36.

Weeks 38-40: full term. Baby ready for delivery. Continues to gain weight until born.

Symptoms — what’s normal, what to call about

Normal first trimester: fatigue, nausea (“morning sickness” — often all day), breast tenderness, frequent urination, food aversions or cravings, mood swings, light spotting around implantation.

Normal second trimester: energy return, growing belly, visible stretch marks, “round ligament pain” in the lower abdomen, nasal congestion, leg cramps.

Normal third trimester: back pain, swelling in feet/ankles, Braxton Hicks contractions (irregular practice contractions), heartburn, difficulty sleeping, shortness of breath, colostrum leaking.

Call the provider immediately for: heavy bleeding, severe abdominal pain, severe headache with vision changes, sudden swelling of face/hands, persistent vomiting (unable to keep fluids down), decreased fetal movement after week 28, fever over 38°C / 100.4°F, contractions before week 37 that don’t stop with rest and hydration, fluid leaking from vagina.

Weight gain guidance

IOM (Institute of Medicine) recommendations, based on pre-pregnancy BMI:

Underweight (BMI < 18.5): 28-40 lb (12.5-18 kg).

Normal weight (BMI 18.5-24.9): 25-35 lb (11.5-16 kg).

Overweight (BMI 25-29.9): 15-25 lb (7-11 kg).

Obese (BMI ≥ 30): 11-20 lb (5-9 kg).

Most gain is in the second and third trimesters (~1 lb / week). Individual variance is high.

Nutrition essentials

Folic acid (400-800 mcg daily) — critical in first trimester to prevent neural tube defects. Start before conception if possible.

Iron (27 mg daily) — plasma volume expands by 50% during pregnancy, diluting hemoglobin. Iron deficiency anemia is common.

Calcium (1,000 mg daily) — skeletal development.

DHA (200 mg daily) — fetal brain development. From fish oil or algae-based supplements.

Protein — an additional 20-25 g/day in second and third trimesters.

Foods to avoid: raw fish, raw/undercooked meat, unpasteurized dairy, deli meats (listeria risk unless heated), high-mercury fish (shark, swordfish, king mackerel, tilefish), alcohol, large amounts of caffeine (>200 mg/day).

What the due date really means

About 25% of first-time parents deliver on or before the due date; 75% after. Only ~5% deliver exactly on the EDD.

Reframe the due date as “sometime in weeks 39-41” rather than a specific day. Planning work, childcare, and visitors with a week-long window reduces stress when the baby arrives at week 40+4 or week 39+2.

Run the numbers

Get due date, current gestational week, and trimester with the pregnancy calculator. Pair with the ovulation calculator if you’re tracking cycles before conception, and the age calculator for exact week/day counts from LMP or conception.