Pregnancy Development Timeline

Overview

This lecture covers fetal development and maternal changes during pregnancy, week by week, from conception through the end of the third trimester.

Weeks 1–2: Egg develops and implants in the uterus.
Week 3: Baby is a blastocyst, several hundred rapidly dividing cells.
Week 4: Embryo consists of two layers; primitive placenta forms; length 0.2 mm.
Week 5: Embryo grows quickly; pregnancy symptoms start; length 2 mm.
Week 6: Facial features begin forming; morning sickness possible; heart activity visible; length 4 mm.
Week 7: Limb buds form; uterus doubles in size; length 1 cm; heart rate 145–180 BPM.
Week 8: Baby is moving; decisions about prenatal tests may begin; length 1.6 cm; weight 1 g.
Week 9: Baby looks more human; waist may thicken; length 2.3 cm; weight 2 g.
Week 10: Major organs and structures complete; length 3.1 cm; weight 4 g.
Week 11: Hands open/close; tooth buds form; length 4.1 cm; weight 10 g.
Week 12: Toes curl; brain and kidney development; length 5.4 cm; weight 14 g.
Week 13: Fingerprints form; nearly 3 in long; length 7.4 cm; weight 23 g.

Week 14: Baby's facial expressions visible; energy improves; length 8.7 cm; weight 43 g.
Week 15: Baby senses light, forms taste buds; stuffy nose possible; length 10.1 cm.
Week 16: Growth spurt begins; length 11.6 cm; weight 100 g.
Week 17: Skeleton ossifies; umbilical cord thickens; length 13 cm; weight 140 g.
Week 18: Genitals visible on ultrasound; increased appetite; length 14.2 cm; weight 190 g.
Week 19: Baby may hear; round ligament pain possible; length 15.3 cm; weight 240 g.
Week 20: Halfway point; baby swallows, makes meconium; length 16.4 cm; weight 300 g.
Week 21: Baby movement felt; eyebrows develop; length 26.7 cm; weight 360 g.
Week 22: Baby resembles a newborn; belly size increases; length 27.8 cm; weight 430 g.
Week 23: Baby feels mother's motion; swelling in ankles/feet; length 28.9 cm; weight 501 g.
Week 24: Uterus soccer ball size; length 30 cm; weight 600 g.
Week 25: Baby gains fat, more hair; mother may notice hair changes; length 34.6 cm; weight 660 g.
Week 26: Baby practices breathing with amniotic fluid; length 35.6 cm; weight 760 g.
Week 27: Hiccups, eye opening, finger sucking; length ~36.6 cm; weight 875 g.

Week 28: Baby perceives light; length 37.6 cm; weight 1050 g.
Week 29: Muscles/lungs mature; head grows; length 38.6 cm; weight 1153 g.
Week 30: Baby almost 3 lbs; mood swings and fatigue common; length 39.9 cm; weight 1319 g.
Week 31: Strong kicks, Braxton Hicks contractions; length 41.1 cm; weight 1502 g.
Week 32: Baby gains fat; possible heartburn and breathlessness; length 42.4 cm; weight 1720 g.
Week 33: Over 4 lbs; sleep discomfort increases; length 43.7 cm; weight 1918 g.
Week 34: CNS and lungs mature; dizziness, fatigue possible; length 45 cm; weight 2146 g.
Week 35: Baby movement less dramatic; length 46.2 cm; weight 2383 g.
Week 36: Baby drops into pelvis; gains about 1 oz/day; length 47.4 cm; weight 2622 g.
Week 37: Brain/lungs mature; more discharge/contractions; length 48.6 cm; weight 2859 g.
Week 38: Firm grasp; monitor for preeclampsia; length 49.8 cm; weight 3083 g.
Week 39: Full-term; contact healthcare provider if water breaks; length 50.7 cm; weight 3188 g.
Week 40: Due date; labor starts or induction considered; length 51.2 cm; weight 3462 g.

Blastocyst — Early-stage embryo, a ball of cells formed after fertilization.
Embryo — Developing baby from implantation to 8 weeks.
Placenta — Organ supplying oxygen/nutrients to the embryo or fetus.
Meconium — Baby's first stool, composed of swallowed materials.
Braxton Hicks contractions — Irregular, usually painless uterine contractions in late pregnancy.
Preeclampsia — Pregnancy complication causing high blood pressure.