Weeks of pregnancy: what happens week by week

Irene · May 24, 2026 · 9 min

When you find out you're pregnant, one of the first things you learn is that pregnancy is measured in weeks, not months. And that the baby grows in extraordinarily different ways week by week — in the first trimester it changes practically every day. This guide summarizes what happens from the 4th to the 40th week, both to your baby and to you, and which milestones to mark on the calendar.

How pregnancy weeks are actually counted

Surprise number one: pregnancy weeks do not start from conception, but from the first day of your last menstrual period (LMP). This means that when you're told you're “in your 6th week,” the baby is actually about 4 weeks of embryonic life: the first two weeks “count” even though there was no embryo yet.

It's an international convention (called gestational age) used by every OB-GYN in the world, because the date of ovulation is almost always uncertain while the last menstrual period is a memorable date. According to Naegele's rule, the due date is calculated by adding 280 days (40 weeks) to the first day of the LMP. If you want to calculate your current week and your estimated due date precisely, you can use the BabyWish pregnancy calculator: all you need is one date.

First trimester (weeks 4 – 13)

It's the period when the most things happen, both for the baby and for you. It's also when symptoms are often most intense.

Weeks 4-6: the discovery

The pregnancy test turns positive. The embryo is the size of a poppy seed (week 4) and grows to the size of a grain of rice (week 6). The neural tube(the base of the nervous system) forms and the heart starts beating: around the 6th week it can already be seen on a transvaginal ultrasound.

You might notice: unexplained tiredness, swollen and sore breasts, first bouts of nausea, frequent need to urinate.

Weeks 7-9: the first shapes

The embryo begins to develop the buds of arms, legs, fingers, eyes, ears. At the 9th week it officially becomes a fetus. Its size goes from 1 cm (week 7) to about 2-3 cm (week 9).

You: nausea may reach its peak. Mood swings, sensitivity to smells, aversion to certain foods. Blood volume starts to increase.

Weeks 10-13: the baby is already visible

The fetus reaches 7-8 cm by the 13th week. Nails, taste buds and external genitals form. It starts making its first movements — you can't feel them yet, but you can see them on ultrasound.

It's during this window (between weeks 11 and 13+6) that the first-trimester ultrasound is performed, essential for:

Confirming the estimated due date (more accurate than the one calculated from the LMP).
Measuring the nuchal translucency, the first indicator of the risk of chromosomal abnormalities.
Seeing the number of embryos (yes, including twins).

Second trimester (weeks 14 – 27)

Often called “the golden phase” of pregnancy: nausea typically decreasing, energy coming back, belly starting to show.

Weeks 14-17: energy returns

The baby is about 15 cm and 150 grams by week 17. It starts to hear sounds, especially mom's voice. Hair begins to grow.

You: your belly button may stick out, the skin can feel a bit tight on the abdomen. It's often the right time to open the baby registry and start sharing it with friends and relatives — here you'll find all the tips for organizing your pregnancy calendar.

Weeks 18-21: the anatomy scan

It's the most important ultrasound of the entire pregnancy, usually performed between weeks 19 and 21. It lasts 30-45 minutes and checks in detail:

Heart anatomy (4 chambers, valves, great vessels).
Brain, spine, kidneys, bladder, stomach.
Length of the limbs, development of the face.
Amount of amniotic fluid and position of the placenta.
Baby's sex — if you want to know.

The baby is now 25-27 cm and 300-400 g. You start to feel the first movements, often described as “bubbles” or little taps (around weeks 18-20 for first-time moms, earlier for those who have already given birth).

Weeks 22-27: the baby finds a rhythm

Movements become more decisive. The baby has its own sleep-wake rhythm, often opposite to yours (sleeps when you walk, awake when you relax). It opens its eyes for the first time around weeks 26-27.

Between weeks 24 and 28 the glucose tolerance test(OGTT) is performed — essential to rule out gestational diabetes.

At week 27 the baby measures about 36 cm and weighs 900-1000 g. It's the theoretical threshold of extreme prematurity: born at this age, it has good chances of survival with specialized neonatal care.

Third trimester (weeks 28 – 40)

The baby completes the development of lungs and brain, accumulates fat and gains weight rapidly. You start preparing the house, the baby registry, the hospital bag.

Weeks 28-31: growing fast

The baby doubles its weight in two months: by week 31 it's already 40 cm and 1.5 kg. It sees light through the abdominal wall and recognizes familiar voices.

It's the right time to start the antenatal class (typically between weeks 28 and 32) and to check the baby registry: balanced prices, collective gift active, link shared with relatives.

Weeks 32-35: position and preparation

The baby usually turns head down between weeks 32 and 34. It continues to accumulate fat, the skin smooths out and becomes less wrinkled. By week 35 it's about 46 cm and 2.4 kg.

You: Braxton Hicks contractions (practice contractions), heavy legs, possible difficulty sleeping. Maternity leave typically begins around week 36 (varies by country).

Weeks 36-40: ready

From week 37, the pregnancy is considered “at term”: the baby can be born at any moment and is fully developed. It measures about 50 cm and weighs 3-3.5 kg by week 40.

Practical milestones in this phase:

End-of-pregnancy check-up at 36-37 weeks: cardiotocography monitoring, position check.
Vaginal-rectal swab for group B strep (weeks 36-37).
Hospital bag ready by week 36.
Car seat installed and checked — it's mandatory for going home.

Beyond week 40

Only 4-5% of babies are born exactly at week 40. Between weeks 40 and 41 it's considered physiological; beyond week 41 it's called prolonged pregnancy and closer monitoring begins (check-ups every 48 hours). Induction of labor is typically proposed between weeks 41 and 42.

The milestones to mark on the calendar

To sum up, these are the key appointments of the pregnancy calendar:

Weeks 6-8 — first OB-GYN visit and dating.
Weeks 11-13+6 — first-trimester ultrasound + nuchal translucency.
Weeks 19-21 — anatomy scan.
Week 22 — ideal time to open and share the baby registry.
Weeks 24-28 — glucose tolerance test.
Weeks 28-32 — start of antenatal class.
Weeks 32-34 — third-trimester ultrasound (growth, position, placenta).
Weeks 36-37 — group B strep swab + hospital bag ready.
Week 37 onward — pregnancy at term, the baby can be born.

In summary

Pregnancy is measured in weeks because every week brings precise changes — in the baby, in your body and in the calendar of exams and choices. Knowing the big picture helps you not to be late on important check-ups and to calmly prepare what you really need: from the baby registry to the antenatal class, from the hospital bag to the car seat.

To know exactly which week you're in, what your estimated due date is and what happens in the coming weeks, you can use the BabyWish pregnancy calculator. Free, no registration, with the complete calendar of your pregnancy on a single screen.