Pregnancy, week by week

Forty-two weeks from last period to due date — what's developing, what's changing, and one small checklist per week, written for body, feeding, postpartum recovery, and close-in care.

First trimester

Week 1Week 1 is counted from the first day of the last menstrual perio…
Week 2Ovulation typically happens near the end of this week; fertiliza…
Week 3The fertilized egg divides as it travels toward the uterus, beco…
Week 4Implantation usually completes this week. The embryo is about th…
Week 5The neural tube — which becomes the brain and spinal cord — is f…
Week 6The heart begins to beat in a regular rhythm, and small buds tha…
Week 7The brain is growing rapidly, arms and legs lengthen, and the em…
Week 8Fingers and toes are forming, the embryo is moving (though it ca…
Week 9All essential organs have begun to form. The embryo is transitio…
Week 10Now officially a fetus. Vital organs are formed and beginning to…
Week 11Genitals begin to develop, tooth buds and hair follicles form, a…
Week 12Reflexes begin — the fetus can curl its toes and make sucking mo…
Week 13The fetus can put a thumb in its mouth, vocal cords are forming,…

Second trimester

Week 14The second trimester begins. The fetus can make facial expressio…
Week 15The fetus senses light through the abdominal wall, taste buds ar…
Week 16Eyes and ears are nearly in their final positions. The fetus is …
Week 17The skeleton is hardening from cartilage to bone, the umbilical …
Week 18Ears move into final position and hearing structures mature. Man…
Week 19A protective coating called vernix caseosa is forming on the ski…
Week 20Halfway by the calendar. The fetus can swallow, produces meconiu…
Week 21Movements become more coordinated — kicks and rolls replace flut…
Week 22Lips and eyebrows are distinct, the eyes have formed (eyelids st…
Week 23Lung blood vessels are developing in preparation for breathing, …
Week 24The lungs continue branching and growing air sacs. Taste buds ca…
Week 25The fetus responds to familiar voices with movement, nostrils ar…
Week 26Lungs are developing surfactant, the substance that helps air sa…
Week 27The second trimester ends. The fetus sleeps and wakes in regular…

Third trimester

Week 28The third trimester begins. Eyelashes have formed, the eyes can …
Week 29Muscles and lungs keep maturing, and the head is growing to make…
Week 30The brain is developing grooves and wrinkles as it grows, and th…
Week 31The fetus can turn its head, process information, and track ligh…
Week 32Toenails and fingernails are fully formed, and the fetus practic…
Week 33The skull bones are still flexible and not yet fused, which help…
Week 34The central nervous system and lungs continue to mature, and the…
Week 35Kidneys are fully developed, the liver can process some waste pr…
Week 36The fetus is gaining weight steadily and shedding most of the do…
Week 37Early term. The fetus practices grasping, turns toward light, an…
Week 38The fetus has a firm grasp and continues gaining — building the …
Week 39Full term. The brain continues developing at a rapid pace, and t…
Week 40The due date arrives — an estimate, not a deadline. Only a small…
Week 41Late term. Providers typically increase monitoring and discuss t…
Week 42Post-term by definition at 42 completed weeks. Most providers re…

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Educational information only — not medical advice. Every pregnancy and every baby develops on their own timeline. If you have any concern, talk to your obstetric provider or your baby's pediatrician.