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.
- 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.
Image credit: Stephen Kelly, 2018
The first trimester is the first
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
- 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.
- tender, swollen breasts
- mood changes
- cravings for certain foods
- a need to urinate more often
- weight changes
- 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.
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
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:
- shortness of breath
- swelling in the ankles, face, and fingers
- mood changes
- leakage of milk from the breasts
- other breast and nipple changes
- frequent urination
- Braxton-Hicks contractions, which do not indicate labor
- real contractions, which indicate labor
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
- 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
If you know someone at immediate risk of self-harm, suicide, or hurting another person:
- Ask the tough question: “Are you considering suicide?”
- Listen to the person without judgment.
- Call 911 or the local emergency number, or text TALK to 741741 to communicate with a trained crisis counselor.
- Stay with the person until professional help arrives.
- Try to remove any weapons, medications, or other potentially harmful objects.
If you or someone you know is having thoughts of suicide, a prevention hotline can help. The National Suicide Prevention Lifeline is available 24 hours per day at 800-273-8255. During a crisis, people who are hard of hearing can use their preferred relay service or dial 711 then 800-273-8255.
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.