Fetal development week by week

If you're trying to decide what to do, it can help to know what has developed so far. Weeks here are counted the way doctors count them, from the first day of your last period. Conception is usually about two weeks after that.

Not sure how far along you are? A free ultrasound at a local pregnancy center can tell you.

First trimester (weeks 1 to 13)

Week	What's developing
3 to 4	The fertilized egg implants in the wall of the uterus. The cells that will become the placenta and the embryo begin to form.
6 to 7	Cardiac activity can usually be seen on a vaginal ultrasound, at a regular rhythm. Buds that will become arms and legs appear.
8 to 9	Hands and feet take shape, and fingers and toes begin to form.
9 to 10	All of the essential organs have begun to grow. They keep developing and maturing for the rest of pregnancy. From week 10, the embryo is called a fetus, and the heartbeat can often be heard with a handheld Doppler.
11 to 13	Fingernails and toenails begin to grow. The structures needed for hearing begin developing around weeks 13 to 16.

Second trimester (weeks 14 to 27)

Week	What's developing
19 to 21	The fetus can hear. Many people first feel fluttering movements, sometimes called quickening.
22	Movement can usually be felt. The heartbeat can be heard with a stethoscope.
22 to 25	Survival outside the womb becomes possible with intensive hospital care. ACOG reports survival to hospital discharge of about 5 to 6% before 23 weeks, 23 to 27% at 23 weeks, 42 to 59% at 24 weeks and 67 to 76% at 25 weeks.
25 to 28	The fetus may startle at loud sounds.

Third trimester (weeks 28 to 40)

The fetus gains most of its weight, and the brain and lungs keep maturing. A pregnancy is full term from 39 weeks through 40 weeks and 6 days.

Where experts disagree

Can a fetus feel pain?

The major medical groups' view: the American College of Obstetricians and Gynecologists (ACOG) says a fetus doesn't have the capacity to feel pain until at least 24 to 25 weeks, because the brain connections it considers necessary develop at or after 24 weeks. The UK's Royal College of Obstetricians and Gynaecologists (RCOG) reviewed the evidence in 2022 and concluded that pain perception before 28 weeks is unlikely. Both say reflex reactions to touch earlier in pregnancy aren't the same as feeling pain.

The other view: in a 2020 paper in the Journal of Medical Ethics, two researchers who hold opposing views on abortion argued that the part of the brain these groups point to may not be required for pain. They concluded the evidence points toward a basic, unreflective pain experience that may be possible from as early as 12 weeks, and that pain relief during later abortions is reasonable to consider.

Pain can't be measured directly in a fetus, and this question is still debated.

Sources

MedlinePlus. Fetal development.
ACOG. How your fetus grows during pregnancy; Understanding and navigating viability; Abortion language guide.
ACOG. Gestational development and capacity for pain.
RCOG. Fetal awareness: evidence review (2022).
Derbyshire SWG, Bockmann JC. Reconsidering fetal pain. Journal of Medical Ethics, 2020.

Common questions

When does a baby have a heartbeat?

Cardiac activity can usually be seen on a vaginal ultrasound at about 6 to 7 weeks of pregnancy. Many people call this the heartbeat. ACOG prefers the term cardiac activity before about 10 weeks, because the heart's chambers are still forming. A doctor can usually hear the heartbeat with a handheld Doppler at around 10 weeks.

What is developing at 8 weeks?

By 8 weeks, cardiac activity can be seen on ultrasound, arm and leg buds have formed, and hands and feet are taking shape, with fingers and toes beginning to form around 8 to 9 weeks.

When can a baby feel pain?

Scientists disagree. ACOG says the brain connections needed to feel pain aren't in place until at least 24 to 25 weeks, and the UK's Royal College of Obstetricians and Gynaecologists says pain before 28 weeks is unlikely. Some researchers argue a basic form of pain can't be ruled out from as early as 12 weeks. See the section on this page for both views.

When do you feel the baby move?

Fluttering movements are usually first felt around 19 to 21 weeks, and movement can usually be felt by about 22 weeks.

Last updated 2026-11-02. Medical facts on this page are drawn from the sources listed at the end and on our sources page. This is general information, not medical advice.