Pregnancy milestones week by week

Using ACOG's convention, the first trimester runs from 0+0 through 13+6. Early in this period a blastocyst implants, the embryo stage begins, and structures that will become the brain, spine, digestive tract, face, inner ear, arms, and legs start forming. These processes overlap; they do not wait for a page to change from one numbered week to the next.

During weeks 9 through 12, cartilage, eyelids, nails, and organ functions continue developing. ACOG's infographic calls week 11 the start of the fetal stage when gestational age is used. A dating scan may be offered during this trimester, but the exact schedule and screening choices depend on country, service, personal circumstances, and consent.

ACOG defines the second trimester as 14+0 through 27+6. Bones harden, movements become more coordinated, hearing develops, and body systems continue maturing. A pregnant person may begin to feel movement between 16 and 24 weeks. In a first pregnancy, that first recognized movement may occur after 20 weeks.

In England, the NHS offers a screening scan usually between 18 and 21 weeks to look for signs of specified conditions and check growth. It cannot find every condition, and it is a choice rather than a guarantee. Other countries use different schedules and terminology, so a global milestone calendar should label the jurisdiction instead of presenting one scan window as universal.

The third trimester begins at 28+0 under ACOG's convention. The fetus continues gaining weight while the brain, lungs, nervous system, bones, and other organs mature. Movement should remain present through the end of pregnancy and during labor, although the kinds of movement may feel different as space and position change.

The third trimester is not one undifferentiated “term” period. ACOG defines early term as 37+0 through 38+6, full term as 39+0 through 40+6, late term as 41+0 through 41+6, and postterm from 42+0. Those labels describe gestational periods; they do not decide an individual's delivery plan.

There is no single correct number of movements per day. NHS guidance recommends learning the baby's usual pattern rather than comparing it with a generic kick target. If no movement has been felt by 24 weeks, tell the midwife. If movement becomes less frequent, stops, or changes from the usual pattern at any stage after it has been established, call immediately.

Do not wait until the next day, even in the middle of the night. Do not use a home Doppler to decide that the baby is well; hearing a heartbeat does not provide that assessment. A tracker may help someone notice their own observations, but it must never delay contact or mark a concerning change as normal for a particular week.

A milestone guide describes what commonly develops during a broad period. It cannot determine fetal health, predict exact size, or prove that a structure is normal. Approximate measurements and illustrated comparisons are population-level teaching aids. Ultrasound findings and the care team's interpretation belong to the individual pregnancy and should not be replaced by a generic week page.

Use an established estimated due date to calculate completed weeks and days, then read the relevant trimester or multiweek range. Avoid creating forty repetitive pages that attach one promise to each week. Broader, sourced articles provide the context needed to discuss variation, scans, movement, term language, and when direct care matters more than the calendar.