Pregnancy Week by Week: What Each Stage Brings

Forty Weeks of Transformation, Measured Day by Day

At 8 weeks of gestation, a human embryo measures about 16 mm, the size of a raspberry. Its heart has been beating since week 6. Its neural tube, which forms the brain and spinal cord, closed by week 4. Most of the critical structural development that determines a child's anatomy for life happens before the pregnant person even schedules their first prenatal appointment. This is why prenatal care begins as early as possible and why the first trimester carries disproportionate clinical significance.

Pregnancy divides into three trimesters, each with distinct developmental milestones, screening opportunities, and clinical priorities. The first trimester (weeks 1-13) involves organ formation and carries the highest miscarriage risk. The second trimester (weeks 14-27) brings detectable fetal movement and the anatomy scan. The third trimester (weeks 28-40) focuses on fetal growth, lung maturation, and delivery preparation. Each trimester has specific windows for genetic screening and prenatal testing that cannot be replicated later if missed.

This guide walks through each trimester's major milestones, explains when and why specific prenatal appointments occur, and clarifies the genetic screening options available at defined gestational age windows.

The Three Trimesters: Boundaries and Key Events

Obstetric care divides pregnancy into three trimesters by gestational age (counted from the first day of the last menstrual period):

• First trimester: Weeks 1-13. Fertilization occurs around week 2 (gestational age). The embryonic period (weeks 3-8) is when all major organ systems form. By week 10, the embryo becomes a fetus, meaning all basic structures are present. The heart, brain, spinal cord, limbs, and facial features are discernible by week 10. Weeks 11-13 mark the end of peak organogenesis risk.
• Second trimester: Weeks 14-27. The fetus grows from about 87 mm (crown-rump) to 35 cm (crown-heel). Fetal movement becomes detectable between weeks 18-22 (quickening). The fetus can hear external sounds by week 20. Lung surfactant begins developing around week 24, which marks the threshold of viability outside the womb with intensive support. By week 26, the fetus can open its eyes.
• Third trimester: Weeks 28-40+. Rapid weight gain, the fetus gains about 200 g per week in this period. Brain development accelerates. Fat deposits build under skin. Lungs mature: surfactant production is complete by week 34, making preterm deliveries after 34 weeks much safer. The fetus settles into a head-down position (vertex presentation) in most cases between weeks 32-36.

Key Developmental Milestones Week by Week

The developmental timeline uses gestational weeks (from LMP):

• Week 4: Neural tube closes. Embryo implants in uterine wall. Home pregnancy tests turn positive (hCG detectable).
• Week 6: Heartbeat detectable by transvaginal ultrasound. Embryo measures about 6 mm.
• Week 8: All major organ systems are forming. Limb buds present. Embryo measures 16 mm.
• Week 10: Embryo transitions to fetus. Fingers and toes distinct. Crown-rump length approximately 31 mm.
• Week 12: Fetus measures about 53 mm. Risk of chromosomal abnormality drops slightly after nuchal translucency screening.
• Week 18-20: Anatomy scan. All major organ systems visualized. Sex can be determined.
• Week 24: Viability threshold, survival possible with intensive NICU support.
• Week 28: Third trimester begins. Fetus weighs approximately 1 kg.
• Week 34: Lung maturation largely complete. Preterm births after 34 weeks carry dramatically lower risk than earlier deliveries.
• Week 37: Early term. Delivery carries low risk from gestational age standpoint.
• Week 39-40: Full term. Optimal delivery window.

Common Misconceptions

• Pregnancy is 9 months long. Full-term pregnancy runs 40 weeks (or 280 days), which is closer to 9 months and 1 week by calendar month reckoning. Some months have 4 weeks, others have 4.3. Forty weeks divided into roughly 13-week trimesters makes each trimester about 3 months and 1 week, not exactly 3 months.
• The first prenatal visit determines the due date. The due date calculates from the last menstrual period, not the first prenatal visit. The first visit (typically at 8-10 weeks) confirms the pregnancy, orders lab work, and conducts a dating ultrasound to verify or adjust the LMP-based date. The date itself was set from the moment the LMP started.