Pregnancy Trimesters: Week-by-Week Guide

"A trimester is a period of 3 months, one third of the 9-month period of gestation. The most critical stages of development occur during the first trimester (13 weeks), when embryonic and early fetal development is occurring." - The Developing Human: Clinically Oriented Embryology

🟢 FIRST TRIMESTER — Weeks 1 to 12

Week 1-2 — Fertilization & Implantation

• Sperm fertilizes the egg in the fallopian tube, forming a zygote
• Rapid cell division (cleavage) forms a morula, then a blastocyst
• Blastocyst implants into the uterine endometrium around day 6-10
• No visible embryo yet; pregnancy test becomes positive around week 4 (LMP)

Week 3 — Gastrulation

• Three germ layers form: ectoderm, mesoderm, endoderm
• Neural tube (future brain and spinal cord) begins forming
• Heart cells start developing
• Embryo is about 1.5-2.5 mm

Week 4 — Organogenesis Begins

• Heart begins beating (cardiac contractions start ~day 22-23)
• Neural tube closes
• Arm and leg buds appear
• Embryo looks like a curved "C" shape, size of a poppy seed (~2-4 mm)

Week 5-6 — Facial Features Begin

• Brain divides into 5 vesicles
• Eyes, ears, and nose start forming as pits/placodes
• Limb buds elongate; paddle-shaped hand plates visible
• Heart has 4 chambers forming
• Size: sesame seed to lentil (~4-6 mm)

Week 7-8 — End of Embryonic Stage

• Fingers and toes are forming (webbed at first)
• Eyelids begin forming
• All major organs are present in rudimentary form
• Embryo develops distinctly human features
• Embryonic development ends at stage 23 (~day 56/week 8)
• Size: raspberry (~15-22 mm)

Week 9-10 — Fetal Period Begins

• Now called a fetus (not embryo)
• Fingers and toes are separated
• External genitalia beginning to differentiate (sex not yet visible on ultrasound)
• Fetus can make small movements
• Swallowing begins; kidneys produce urine
• Size: strawberry (~30-40 mm)

Week 11-12 — End of First Trimester

• All organ systems present and functioning at a basic level
• Facial features more refined; eyelids fused
• Fingernails forming
• Risk of miscarriage drops significantly after this point
• Size: plum (~50-60 mm); weight ~14-45 g
• Nuchal translucency ultrasound done at 11-13 weeks

🟡 SECOND TRIMESTER — Weeks 13 to 27

Week 13-16 — Rapid Growth

• Skeleton ossifying (hardening from cartilage to bone)
• Fine hair (lanugo) begins to appear
• Meconium (first stool) begins accumulating
• Fetus starts making sucking movements
• Sex can often be determined on ultrasound by week 16
• Size: lemon to avocado (~75-115 mm)

Week 17-20 — Quickening (First Movements Felt)

• Mother begins to feel fetal movements ("quickening") around weeks 18-20
• Vernix caseosa (protective white coating) covers the skin
• Eyebrows and eyelashes forming
• Fetus can hear sounds from outside the womb
• Anatomy ultrasound (anomaly scan) done at 18-20 weeks
• Size: banana (~16 cm); weight ~300 g

Week 21-24 — Viability Zone

• Skin appears reddish, wrinkled, with visible veins through translucent skin
• Eyelids begin to open (week 26)
• Fetus responds to sounds with movement and increased pulse
• Lungs producing surfactant (from ~week 24)
• Viability threshold: ~22-24 weeks - fetus may survive with intensive care if born now
• By end of week 24: ~30 cm long, ~600 g

Week 25-27 — End of Second Trimester

• Brain developing rapidly; sleep-wake cycles appear
• Fat deposits beginning to form under the skin
• Responds to pain and light
• Amniotic fluid begins to diminish slightly
• Size: ~34-37 cm; weight ~900 g-1 kg

🔴 THIRD TRIMESTER — Weeks 28 to 40

Week 28-32 — Brain & Lung Development

• Brain develops most rapidly in this period
• Fetus can see and hear
• Most internal systems well-developed; lungs still maturing
• Fetus changes position frequently; may turn head-down (cephalic)
• Weight: ~1.5-2.5 kg; length ~40-45 cm
• If born premature at 28-32 weeks, survival rate is high with NICU care

Week 33-36 — Lung Maturation

• Lungs close to fully developed by week 36
• Brain and nervous system continue developing
• Lanugo (fine hair) begins to disappear
• Toenails and fingernails fully formed
• Weight: ~2.5-3.5 kg; length ~45-48 cm
• Fetus settles into head-down position for birth (engagement)

Week 37-40 — Term

• Week 37+ = Early term; Week 39-40 = Full term
• Lungs fully mature, surfactant production complete
• Vernix mostly absorbed into skin
• Skull bones still soft and unfused (fontanelles) to allow passage through birth canal
• Fetus mostly full-grown; puts on ~30 g/day in final weeks
• Weight: ~3-4 kg; length ~48-52 cm
• Labor usually begins between weeks 39-41

Summary Table

Key Prenatal Visits & Screenings by Trimester