Trimesters of Pregnancy: How Many Weeks Each One Lasts and What Happens

How many weeks are in each trimester?

ACOG (the American College of Obstetricians and Gynecologists) defines the first trimester as the first day of your last period to 13 weeks and 6 days, the second trimester as 14 weeks 0 days to 27 weeks 6 days, and the third trimester as 28 weeks 0 days to 40 weeks 6 days. Cleveland Clinic describes each trimester as about 13 weeks, or three months, long.

The NHS draws the first line a week earlier. Its week-by-week guide says week 13 marks the start of the second trimester, and that you enter the third trimester in week 28. MedlinePlus, the US National Library of Medicine's health site, starts the second trimester at week 14 and runs it through week 28.

How are the weeks of pregnancy counted?

Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. ACOG says a normal pregnancy lasts about 40 weeks from that date and is assumed to start 2 weeks after it, so the first 2 weeks of the count fall before you are actually pregnant.

Pregnancy can be counted in weeks and sometimes days. ACOG explains that "36 and 3/7 weeks" means 36 weeks and 3 days.

If you are not sure when your last period started, ACOG says a first-trimester ultrasound can estimate the due date from the length of the embryo or fetus; after IVF, the due date is set from the age of the embryo and the date it was transferred. The due date is an estimate either way: ACOG says only about 1 in 20 women give birth on it.

How many months is 40 weeks of pregnancy?

Forty weeks is between nine and 10 months, depending on how you count. Cleveland Clinic describes a full-term pregnancy of 40 weeks as between nine and 10 months, and ACOG's month-by-month chart uses 4-week months, which gives 10:

• Months 1 to 2: weeks 1 to 8
• Month 3: weeks 9 to 12
• Month 4: weeks 13 to 16
• Month 5: weeks 17 to 20
• Month 6: weeks 21 to 24
• Month 7: weeks 25 to 28
• Month 8: weeks 29 to 32
• Month 9: weeks 33 to 36
• Month 10: weeks 37 to 40

Most calendar months have 30 or 31 days, a few more than 4 weeks, so 40 weeks (280 days) comes to a little over nine calendar months. MedlinePlus notes that your doctor or midwife may talk about your pregnancy in weeks rather than months or trimesters.

What happens in the first trimester?

The first trimester is when the baby's major organs form. ACOG describes it as the time when fertilization and major organ development occur, and its chart says that by week 13 all major organs have formed. By the end of week 12, the fetus is about 2 inches (5 cm) long and weighs about half an ounce (14 g).

The changes ACOG lists for your body in these weeks include a missed period, larger and more tender breasts, needing to pee more often, tiredness, nausea and vomiting, cravings or loss of appetite, heartburn, bloating and gas, constipation, and gaining or losing a few pounds.

In the UK, the NHS says the booking appointment usually takes place between weeks 8 and 12, and the dating scan is usually at 8 to 14 weeks. What happens at the first prenatal appointment has its own article.

What happens in the second trimester?

The second trimester is the time of rapid growth and development, in ACOG's words. Its chart says the fetus is more than 6 inches (15 cm) long by the end of week 20, and about 12 inches (30 cm) long and about 1½ pounds (680 g) by the end of week 24.

ACOG says your appetite increases and nausea and tiredness may ease, your abdomen begins to expand, and you begin to feel the fetus move. Other changes on its list include skin on the abdomen that stretches and may feel tight and itchy, an ache on one side or the other as the ligaments that support the uterus stretch, brown patches on the face, darker areolas, and swollen feet and ankles. The week a bump becomes visible varies a lot; when you start showing covers it.

MedlinePlus says you will have a prenatal visit every month in the second trimester. Its list of tests for this stage includes an ultrasound around 20 weeks to check the baby's anatomy (see the 20-week anatomy scan) and glucose tolerance testing for diabetes that can begin during pregnancy.

What happens in the third trimester?

In the third trimester, the fetus gains weight and the organs mature so they are ready to work after birth. ACOG's chart says that at week 31 major development is finished and the fetus is gaining weight very quickly, that most fetuses turn head-down during week 36 or 37, and that by the end of week 40 the baby is 20 inches (51 cm) long and may weigh 7½ to 8 pounds (3.4 to 3.6 kg).

For you, ACOG lists feeling the movements strongly, shortness of breath, needing to pee more often as the fetus drops, colostrum leaking from the nipples, a navel that sticks out, and contractions, which it says can signal false or real labor.

MedlinePlus says third-trimester visits are every 2 weeks until week 36 and then every week, and that a few weeks before the due date your doctor or midwife will test for group B strep.

When is a pregnancy full term?

ACOG and the Society for Maternal-Fetal Medicine define full term as 39 weeks 0 days to 40 weeks 6 days. It is one of four labels they use for babies born at 37 weeks or later:

• Early term: 37 weeks 0 days to 38 weeks 6 days
• Full term: 39 weeks 0 days to 40 weeks 6 days
• Late term: 41 weeks 0 days to 41 weeks 6 days
• Postterm: 42 weeks 0 days and beyond

These replaced the older single label "term" for the whole stretch from 3 weeks before to 2 weeks after the due date. ACOG's committee opinion explains that research showed newborn outcomes, especially breathing problems, vary depending on when within that 5-week window a baby is born.