What Is a Trimester?

Understanding the Stages of Pregnancy

Pregnancy comes with huge changes for you and baby. Your little one undergoes a lot of growth and development throughout the pregnancy in preparation for living outside the womb. Pregnancy is divided into trimesters to easily track that development.

What is a Trimester?

Pregnant woman lying in garden.

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A trimester is a stage of pregnancy. You go through three trimesters, or stages, in pregnancy. Each trimester covers specific weeks. The trimester concept helps define the development that your baby goes through during the pregnancy.

How Long do Trimesters Last?

A full-term pregnancy lasts for 40 weeks, starting with the date of your last period. Each trimester lasts roughly three months. The first trimester is from zero to 12 weeks. The second trimester covers weeks 13 to 27. The third trimester runs from 28 to 40 weeks, or whenever your baby is born. Some charts use slightly different start and end dates for each trimester, but the general time range is the same.

First Trimester

Pregnant woman lying in garden.

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Your 40 weeks of pregnancy starts on the first day of your last period, even though conception doesn't happen until two weeks later. Once the egg is fertilized, it begins growing quickly into a mass of cells that eventually implant in your uterine wall. Hormonal and physical changes begin with implantation.

Weeks three to eight cover the embryonic stage. At week nine, your embryo officially becomes a fetus. The brain, spinal cord and heart begin forming. By eight weeks, the heart pumps and all major organs and external body structures are starting to develop. By the end of the first trimester, your baby is around 3 inches long and weighs almost an ounce.

You also go through major changes. The increasing hormone levels cause pregnancy symptoms to begin. Common signs that you're pregnant include fatigue, breast changes, morning sickness, cravings, food aversions, mood swings, increased urination, constipation, headache and heartburn.

Second Trimester

Welcome to what some people call the honeymoon trimester. Most women finally get a break from the morning sickness and other uncomfortable symptoms of the first trimester. You may notice you have more energy than you had in the first trimester. Your baby belly really starts to pop during the second trimester. Most women start wearing maternity clothes sometime during this period.

Another exciting milestone for mom-to-be is feeling the baby move for the first time. Most women feel the movement between 18 and 22 weeks, but you may feel it later if you're a first-time mom. Those early movements sometimes feel like bubbles or indigestion, but they steadily get stronger until you feel your baby move all the time.

Your baby undergoes more major development during this trimester as well. Eyebrows, eyelashes, fingernails and toenails form. By 20 weeks, your baby can hear. Around the same time, your doctor does a detailed anatomy ultrasound to check the baby's development. He starts gaining body fat and weight. At the end of the second trimester, your baby weighs roughly 1.5 pounds and measures about 10 inches long.

Third Trimester

Your third trimester covers the rest of your pregnancy up to your delivery day. While 40 weeks is the number used to describe pregnancy length, you can deliver anywhere between 37 and 42 weeks and still be considered full term. If you haven't taken childbirth classes yet, the third trimester is the time to do it.

The baby continues growing and preparing to make her big appearance during the third trimester. The organs are already developed, but they continue to mature, so they function properly after birth. Your baby's bones develop by 32 weeks, although they're still soft. Practice breathing also starts around the same time. As your baby nears 36 weeks, she's still active, but she's running out of space, so she may move around less.

Your doctor begins monitoring the baby's positioning to ensure she moves into the heads-down position before birth. Your appointments also involve cervix checks to determine when you start dilating.

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