The three trimesters

How pregnancy is counted

A pregnancy lasts about 40 weeks, counted from the first day of the last menstrual period. Conception usually happens about 2 weeks after that, so the first 2 weeks counted aren't really pregnancy at all.¹ The estimated due date is 40 weeks from that day, but only about 1 in 20 people give birth on it.¹

For the first 8 weeks after fertilization, the pregnancy is called an embryo. After that, it's a fetus.¹ The placenta, which grows from the same early cells, passes oxygen, food and hormones to the and carries waste away.¹

First trimester: up to 13 weeks

This is when fertilization happens and the major organs form.¹ By 8 weeks the brain and spine have begun to form and the heart tissue is developing. By 12 weeks the kidneys make urine and fingernails have formed.¹

Many people feel very tired early in pregnancy.²

Second trimester: 14 to 27 weeks

A time of fast growth.¹ Bones harden, hearing develops, and by 20 weeks the digestive system works. Kicks and turns get stronger from about 21 weeks.¹

Third trimester: 28 weeks to birth

The gains weight and the organs mature so they're ready to work after birth.¹ The lungs make a substance needed for breathing, the eyes sense light, and many babies turn head-down in the last weeks.¹ Tiredness often returns late in pregnancy.²

Prenatal care

Start prenatal care as soon as you think you're pregnant.² At the first visit, expect questions about your last period and health, a physical exam, and urine and blood tests, including some STI tests. Visits get more frequent as pregnancy goes on.²

Eating and drinking

ACOG advises:²

Limit caffeine to less than 200 mg a day, about one 12-ounce cup of coffee.
Avoid high-mercury fish: bigeye tuna, king mackerel, marlin, orange roughy, shark, swordfish and tilefish. Other cooked fish is healthy; aim for two servings a week.
Avoid raw fish, unpasteurized milk and cheese, and heat hot dogs and lunch meats until steaming, because of the risk of listeria.
No alcohol, tobacco, vaping or cannabis. No amount of cannabis is known to be safe in pregnancy.

Recap

Pregnancy is counted from the first day of the last period, so it includes about 2 weeks before conception.
In the first trimester, up to 13 weeks, the major organs form.
In the second trimester, 14 to 27 weeks, the fetus grows fast and kicks get stronger.
In the third trimester, from 28 weeks, the fetus gains weight and the organs mature.
Only about 1 in 20 babies arrive on their due date.

Words to know 5

due date	The estimated day of birth, 40 weeks after the first day of the last period.
embryo	What a pregnancy is called for the first 8 weeks after fertilization.
fetus	What a pregnancy is called from 9 weeks after fertilization until birth.
placenta	The organ that grows in the uterus and passes oxygen and food to the fetus.
trimester	One of the three parts of pregnancy, each about three months long.