Pregnancy Week by Week Guide

Pregnancy is dated from the first day of your last menstrual period (LMP) — not from conception. This convention is used because LMP is a date most women can recall reliably, while conception itself usually happens around two weeks later (at ovulation). So at "4 weeks pregnant," conception was roughly two weeks ago, implantation was a few days ago, and a pregnancy test is just becoming positive. The actual embryonic age is always about two weeks behind the gestational age.

A full-term pregnancy runs about 40 weeks from LMP, with the term "due date" reflecting that week 40 milestone. ACOG defines early term as 37+0 to 38+6, full term as 39+0 to 40+6, late term as 41+0 to 41+6, and post-term as 42+0 and beyond. Only about 4-5% of babies actually arrive on the calculated due date; most arrive within two weeks on either side of it. If a first-trimester ultrasound disagrees with LMP-based dating by more than the ACOG-defined threshold for that gestational age, the ultrasound date is preferred.

The 40 weeks divide into three trimesters: first trimester (weeks 1-12) is when organ systems form and miscarriage risk is highest; second trimester (13-27) is when the anatomy ultrasound, gestational diabetes screening, and quickening happen; third trimester (28-40)is when surveillance for preeclampsia, GBS screening, kick counts, and preparation for labor become the focus. The week-by-week pages below cover what's happening for the baby, what symptoms are common, what visits or tests are typical, and what red-flag signs warrant a call to your provider.

These pages are educational summaries — they don't replace your prenatal care team. If something on this site contradicts what your provider has told you, ask your provider. Every pregnancy has individual considerations (medical history, prior pregnancies, current medications, family genetics) that a generic week-by-week guide can't weight properly.