Pregnancy Weeks Calendar — Full 40-Week Timeline

Month 1 to 2 — Weeks 1 to 8

The first two months of pregnancy correspond to weeks 1 through 8 and are dominated by implantation and organ formation. Weeks 1 and 2 occur before fertilization and represent the menstrual period and follicular phase. By week 3, ovulation and fertilization happen, and the fertilized egg travels down the fallopian tube toward the uterus. Implantation occurs around day 6 to 10 after fertilization, triggering hCG production and a missed period. By week 4, most home pregnancy tests become positive. Weeks 5 through 8 are the most critical period for fetal development because all major organ systems form during this window of organogenesis. The heartbeat is visible by ultrasound around week 6. Folic acid supplementation of 400 to 800 mcg daily is most impactful during this period, reducing neural tube defect risk by up to 70%.

Month 3 to 4 — Weeks 9 to 16

Weeks 9 through 16 bridge the end of the first trimester and the beginning of the second. By week 9, the embryo becomes a fetus, and the period of highest vulnerability to birth defects is largely over. The nuchal translucency (NT) scan, performed between weeks 11 and 14, screens for chromosomal abnormalities. Morning sickness typically peaks around week 9 and begins to ease by weeks 14 to 16 as the placenta takes over hormone production. By week 12, miscarriage risk drops below 3%, and the first prenatal visit is typically completed. The fetus reaches about 11.6 cm (4.6 inches) by week 16. Energy levels usually rebound significantly as nausea fades, and many individuals report a renewed sense of wellbeing in the early weeks of the second trimester.

Month 5 to 6 — Weeks 17 to 24

Weeks 17 through 24 are often the highlight of pregnancy. Quickening — first felt between weeks 16 and 20 — marks the beginning of a physical connection with the baby. The detailed anatomy scan, scheduled between weeks 18 and 22, examines fetal structures in detail and can detect approximately 70% of major structural abnormalities. The sex of the baby is often visible on this scan. By week 21, the baby can hear sounds from the outside world, and parents may begin talking or playing music for the fetus. By week 24, the baby is considered potentially viable with intensive NICU support, and survival rates exceed 60%. The glucose screening test for gestational diabetes is typically performed between weeks 24 and 28.

Month 7 to 8 — Weeks 25 to 32

Weeks 25 through 32 are characterized by rapid fetal growth and preparation for life outside the womb. Between weeks 25 and 28, surfactant production accelerates, the brain develops gyri and sulci, and the baby begins distinguishing between light and dark. Weight gain peaks in this window, with the fetus gaining approximately 200 to 250 grams per week. By week 28, kick count monitoring begins, and the baby weighs about 1 kg (2.2 lb). Between weeks 28 and 32, the baby's circadian rhythm develops, the eyes can open and close, and hearing becomes more refined. Braxton Hicks contractions become more noticeable, and some individuals begin experiencing shortness of breath as the uterus pushes against the diaphragm. The GBS screening swab is typically scheduled between weeks 35 and 37, though appointments may be made during this window.

Month 9 — Weeks 33 to 40

The final month of pregnancy, weeks 33 through 40, is when the baby prepares for birth. By week 34, lung maturity is typically achieved and surfactant levels are adequate for breathing. Between weeks 33 and 36, the baby typically engages — dropping the head into the pelvis — and Braxton Hicks contractions become more frequent. At week 37, the baby is considered early-term, and full-term status is reached at week 39. By week 40, the average baby weighs about 3.4 kg (7.5 lb) and measures about 50 cm (20 inches). The cervix begins to soften and efface, bloody show may appear, and contractions may begin. ACOG recommends that prenatal visits occur weekly from week 36 onward, and the provider will discuss birth plan preferences, warning signs, and hospital logistics. Only about 4 to 5% of babies are actually born on their exact due date.

Post-Term Considerations

If the pregnancy extends beyond the due date, active management becomes important. Late-term pregnancy spans weeks 41 0 days through 41 6 days, and post-term pregnancy begins at 42 weeks 0 days. After 41 weeks, the placenta's nutrient and oxygen transfer efficiency declines, amniotic fluid volume may decrease, and the risks of macrosomia, meconium aspiration, and stillbirth rise measurably. ACOG recommends non-stress tests and amniotic fluid index assessments weekly between 41 and 42 weeks. Induction of labor is strongly recommended at or before 42 weeks to avoid the significantly elevated risks of post-term pregnancy. Approximately 8% of pregnancies reach 41 weeks or beyond, and most resolve with safe induction.