A full-term pregnancy has three trimesters and lasts around 40 weeks — starting from the first day of the last menstrual period. In each trimester, the fetus meets specific developmental milestones.

While 40 weeks is the usual time frame, a full-term baby can be born as early as 37 weeks and as late as 42 weeks.

The Office on Women’s Health defines the three trimesters as follows, though the timing can vary:

  • first trimester: 1–12 weeks
  • second trimester: 13–28 weeks
  • third trimester: 29–40 weeks

Some people also talk about a fourth trimester, which is the 3-month transitional period after delivery.

Read on for more information about what to expect during each pregnancy trimester.

Pregnancy trimester infographic

Image credit: Stephen Kelly, 2018

The first trimester is the first 12 weeks of the pregnancy, and it is a crucial time for fetal development.

The fetus

At conception, the egg and sperm combine to form a zygote, which implants in the wall of the uterus. The zygote becomes an embryo as its cells divide and grow.

By the end of the first 12 weeks:

  • All the body’s major organs and structures have begun to develop.
  • The heart is beating regularly.
  • Fingers and toes have formed.
  • The fetus is around 3 inches (in) long and weighs nearly 1 ounce.
  • The nerves and muscles work together, and the fetus can make a fist.
  • The eyelids have formed and will remain closed until around week 28, to protect the eyes.

The pregnant person

A person also experiences many changes during their first trimester of pregnancy.

These include:

  • fatigue
  • tender, swollen breasts
  • mood changes
  • cravings for certain foods
  • headaches
  • indigestion
  • a need to urinate more often
  • weight changes
  • constipation
  • nausea, sometimes with vomiting, known as morning sickness

Morning sickness can last throughout the first trimester and sometimes beyond. Despite its name, it does not occur only in the morning.

Weeks 14–27 are the second trimester. The fetus goes through many changes during this time, growing to be around 1 foot long and weighing 1.5 pounds.

The fetus

By the end of the second trimester, the following will have happened:

  • Meconium, the first bowel movement, has developed in the intestines.
  • The fetus can see, hear, make a sucking motion, and scratch itself.
  • Skin, hair, and nails have formed.
  • The lungs have formed but do not yet work.
  • The fetus is sleeping and waking regularly.
  • A male’s testicles will have moved to the scrotum, and a female’s eggs will have formed in the ovaries.
  • Taste buds have formed.
  • Bone marrow is making blood cells.
  • Lanugo, which is fine hair, covers the body.

The pregnant person

Many people feel more comfortable during the second trimester of pregnancy. Morning sickness and fatigue often reduce or disappear.

Meanwhile, new changes take place:

  • The abdomen expands as the fetus grows.
  • Stretch marks may appear on the abdomen, thighs, breasts, and buttocks.
  • The areola, the skin around the nipples, becomes darker.
  • The skin on the face and may darken in patches.
  • The ankles, fingers, and face may swell.
  • Itching may occur. If it happens with vomiting or yellowing of the skin or whites of the eyes, known as jaundice, seek medical advice.

It becomes possible to feel the baby’s movements as this trimester progresses.

The third trimester lasts from week 29 until delivery, which is usually around week 40.

The fetus

Most organs and body systems have formed by now, and they will continue to grow and mature.

During this trimester:

  • The bones are hardening.
  • Movements become more noticeable.
  • The eyes are open and can sense light.
  • Lung formation becomes complete.
  • Lanugo falls away, and a waxy coating, called vernix, develops.

Toward delivery, the fetus drops lower in the person’s abdomen and usually turns head-down.

The pregnant person

The growth of the fetus can cause new discomfort at this time.

A person might also experience:

It is also normal to feel anxiety about the delivery and parenthood toward the end of a pregnancy.

The 3 months after delivery play a key role in the health of the person and their baby. Some people call this transitional period a fourth trimester.

While this can be an exciting time, the range of hormonal and environmental changes can pose challenges.

These challenges might involve:

  • recovering after delivery, especially if there are stitches
  • dealing with lochia, a discharge of blood and tissue, which may continue for several weeks
  • cramping, which may feel like menstrual cramping, especially during breastfeeding
  • adjusting to a new role of parenthood
  • learning new skills
  • having sore breasts and other difficulties related to breastfeeding
  • having fatigue, due to a loss of sleep and other factors
  • in some cases, experiencing postpartum depression

Some tips for managing include:

  • limiting visitors, if this helps
  • asking others for help
  • reducing housekeeping duties
  • resting when the baby does
  • eating regularly, as far as possible
  • raising any concerns about the baby, breastfeeding, or personal health
  • attending all followup appointments

Anyone who experiences a persistent low mood, feelings of guilt or inadequacy, or thoughts of harming themselves or the baby should seek immediate medical care and guidance. These can be signs of postpartum depression.

Pregnancy, childbirth, and the first few months with a newborn are unlike any other time in life. They are full of new experiences, great uncertainty, upheavals, and many new emotions.

Getting regular prenatal care is vital during each trimester. A doctor can help ensure that the fetus is meeting their developmental milestones and that the pregnant person is in good health. They can also provide guidance and resources for support.