The First Trimester of Pregnancy

Medically reviewed by Debra Rose Wilson, PhD, MSN, RN, IBCLC, AHN-BC, CHT on November 9, 2017Written by Jacquelyn Cafasso

What is the first trimester?

A pregnancy lasts for about 40 weeks. The weeks are grouped into three trimesters. The first trimester is the time in between fertilization of the egg by the sperm (conception) and week 12 of a pregnancy.

A woman's body goes through many changes during the first 12 weeks of a pregnancy. Women often start to have concerns over:

  • what to eat
  • which types of prenatal tests they should consider
  • how much weight they might gain
  • how they can make sure their baby stays healthy

Understanding a pregnancy week by week can help you make informed decisions and prepare for the big changes that lie ahead.

What happens to a woman's body during the first trimester?

In the first trimester, a woman's body goes through many changes. The body releases hormones that affect almost every single organ in the body. The first sign you may be pregnant is missing a period. As the first few weeks pass, some women experience the following:

You may need to rest more or eat smaller meals during this time. Some women, however, don’t feel any of these symptoms at all.

What happens to the fetus during the first trimester?

The first day of your pregnancy is also the first day of your last menstrual period. At about 10 to 14 days after, an egg is released, combines with a sperm, and conception occurs. A baby develops rapidly during the first trimester. The fetus begins to develop a brain and spinal cord, and the organs begin to form. The baby's heart will also begin to beat during the first trimester.

Arms and legs begin to bud in the first few weeks, and by the end of eight weeks, fingers and toes start to form. By the end of the first trimester, the baby's sex organs have formed. According to the , the baby is now about 3 inches long and weighs almost 1 ounce.

What can be expected at the doctor?

When you first learn you are pregnant, make an appointment with your doctor to begin caring for the developing baby. If you are not already on prenatal vitamins, start them immediately. Ideally, women take folic acid (in prenatal vitamins) for a year before the pregnancy. Women normally see their doctor once a month during the first trimester.

During your first visit, a doctor will take a full health history and perform a full physical and pelvic exam. The doctor may also:

  • perform an ultrasound to confirm the pregnancy
  • perform a Pap test
  • take your blood pressure
  • test for sexually transmitted infections, HIV, and hepatitis
  • estimate your date of delivery or "due date," which is around 266 days from the first day of your last period
  • screen for risk factors like anemia
  • check thyroid levels
  • check your weight

At around 11 weeks, the doctor will perform a test called a nuchal translucency (NT) scan. The test uses an ultrasound to measure the baby's head and thickness of the baby’s neck. The measurements can help determine the chance that your baby will be born with a genetic disorder known as Down syndrome.

Ask your doctor whether or not genetic screening is recommended for your pregnancy. Genetic screening is a test used to find out your baby's risk for specific genetic diseases.

How can I stay healthy during the first trimester?

It’s important for a woman to be aware of what to do and what to avoid while pregnant in order to take care of themselves and their developing baby.

What to do

Here are good personal health measures to take during the first trimester:

  • Take prenatal vitamins.
  • Exercise regularly.
  • Work out your pelvic floor by doing Kegel exercises.
  • Eat a diet high in fruits, vegetables, low-fat forms of protein, and fiber.
  • Drink lots of water.
  • Eat enough calories (about 300 calories more than normal).

What to avoid

These things should be avoided during the first trimester:

  • strenuous exercise or strength training that could cause an injury to your stomach
  • alcohol
  • caffeine (no more than one cup of coffee or tea per day)
  • smoking
  • illegal drugs
  • raw fish or smoked seafood (no sushi)
  • shark, swordfish, mackerel, or white snapper fish (they have high levels of mercury)
  • raw sprouts
  • cat litter, which can carry a parasitic disease called toxoplasmosis
  • unpasteurized milk or other dairy products
  • deli meats or hot dogs

What else should be considered during the first trimester?

Body changes provide plenty to think about during the first trimester, but having a baby will affect other parts of your life too. There are many things to start to think about during the first few months of your pregnancy so you can prepare for the future.

When to tell your friends, family, and employer

The first trimester is the most common time for a loss of pregnancy (miscarriage), so you may want to wait for the pregnancy to settle into the second trimester.

You may also want to consider whether or not you will keep working or quit your job as your pregnancy progresses, and if your employer provides unpaid maternity leave for the birth and care of your newborn.

Where you want to give birth

You may want to start to consider where you would like to deliver your baby when it’s time to give birth. Women can choose to deliver at a hospital, birth center, or at their own home. You should weigh the pros and cons of each location and discuss them with your doctor.

The (ACOG) believes that hospitals and birthing centers are the safest place to deliver a baby. If there is an emergency, a hospital is fully equipped to handle the situation.

If you have a high-risk pregnancy

High-risk pregnancy means that there is a greater chance of complications. Factors that may make your pregnancy high-risk include:

  • being young
  • being over 35 years old
  • being overweight
  • being underweight
  • having high blood pressure, diabetes, HIV, cancer or other autoimmune disorders
  • being pregnant with twins or multiples

Women with a high-risk pregnancy may need to visit the doctor more often and sometimes may need a specially trained doctor. Having a high-risk pregnancy doesn't necessarily mean you will have any problems.

Paying for care

Many women worry about the costs of medical bills during a pregnancy. The good news is that there are options available in every state in the United States to help pay for care. As soon as you find out you are pregnant, you should make an appointment to see your health care provider, a midwife or a physician (in some medical practices, both are in the same office). Health insurance options have changed over time, and most offer pregnant women more options. Insurance companies are learning it is important to provide prenatal care to prevent more expensive medical care later. Local hospitals, clinics, and other government programs are available to help with:

  • food
  • nutrition
  • counseling
  • free access to health services for pregnant women
CMS Id: 15883