Pregnancy Week by Week: Chart of What Happens

Find the week you are in, then read down the chart. Everything below is from the MedlinePlus fetal development entry and the Office on Women’s Health prenatal testing schedule — no week-by-week fruit comparisons, just what develops and what gets tested.

What develops, and when

Weeks	What happens
1 to 2	Not pregnant yet. The count starts with the period; ovulation comes at the end of week 2
3	Fertilization, then a few days travelling down the fallopian tube as a ball of cells
4	The blastocyst implants in the uterine wall
5	The embryonic period opens: brain, spinal cord, heart and gut begin to form
6 to 7	Arm and leg buds; the heart beats at a regular rhythm, visible on transvaginal ultrasound
8 to 9	Lungs start to form, elbows develop, all essential organs have begun growing
10	No longer an embryo but a fetus; heart tones detectable by Doppler
11 to 14	Face well formed, nails appear, tooth buds form; the head is about half the body
15 to 18	Bones harden, the baby begins to move and stretch, sucking motions appear
19 to 21	The baby can hear; the mother may feel the first fluttering, called quickening
22	Movements clearly felt; the heartbeat can be heard with a stethoscope
23 to 26	Fat storage begins; air sacs form in the lungs but cannot yet work outside the womb
27 to 30	Rapid brain growth; the lungs produce surfactant
31 to 34	Fast weight gain; bones fully developed but still soft; iron and calcium stored
35 to 37	About 5.5 lb (2.5 kg), with definite sleeping patterns and completed muscles
38 to 40	Coarse hair, nails past the fingertips — and 38 weeks since conception, not 40

That last row is worth pausing on. MedlinePlus states it plainly: in the 40th week of pregnancy, it has been 38 weeks since conception. The two extra weeks are the gap between the period the count starts from and the ovulation that follows.

The testing calendar

Prenatal appointments are scheduled around windows, and the windows are defined in weeks:

Weeks	Test
10 to 13	Chorionic villus sampling, if indicated
11 to 14	First trimester screen — blood test plus nuchal translucency ultrasound
14 to 20	Amniocentesis, if indicated
18 to 20	Anatomy ultrasound: organs, body systems, growth, age confirmation
26 to 28	Glucose challenge screening, then a glucose tolerance test if positive
third trimester	Biophysical profile, when the baby’s wellbeing needs monitoring

A note on that glucose row: the Office on Women’s Health puts both the challenge screen and the tolerance test at 26 to 28 weeks, a slightly narrower window than the 24-to-28 many pregnancy calendars quote. Your practice may run it earlier if there are risk factors.

The last few weeks are not padding

The chart ends at 40, but the interesting boundary is 39. NICHD built a national campaign around it because “term” used to cover 37 through 42 weeks as if those weeks were interchangeable. They are not: a baby’s lungs, liver and brain go through a crucial period of growth between 37 and 39 weeks, and babies born before 39 weeks face higher rates of breathing and feeding problems, temperature control problems and NICU admission.

Which is why the number your appointments run on is the week, not the month. To find it, use the pregnancy weeks calculator; for the block it falls in, the pregnancy trimester calculator; and for how the date itself was set, the gestational age calculator.

This content is informational. Every pregnancy has its own pace, and the schedule above is a reference, not your appointment book.