You may be in the first trimester between conception and week 12 of pregnancy. You can take many steps in these early days of pregnancy, but you may also want to avoid certain activities.

I would change this to “The first trimester is the time between conception and 12 weeks of pregnancy.

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 person’s body goes through many changes during the first 12 weeks of pregnancy. You 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.

In the first trimester, the 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, you may experience the following:

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

The first day of your pregnancy is also the first day of your last menstrual period. 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 8 weeks, fingers and toes start to form. By the end of the first trimester, the baby’s sex organs have formed. According to the Office on Women’s Health, the baby is now about 3 inches long and weighs almost 1 ounce.

When you first learn you are pregnant, make an appointment with your doctor or midwife to begin caring for the developing baby. If you are not already on prenatal vitamins, start them immediately.

You typically see your doctor or midwife once a month during the first trimester.

During your first visit, a doctor or midwife will take a full health history and perform a full physical and pelvic exam. They 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 280 days from the first day of your last period
  • screen for risk factors like anemia
  • check thyroid levels
  • check your weight

There are genetic screening options that your doctor or midwife will discuss with you to help determine if your baby is at risk of specific chromosomal abnormalities, such as Down Syndrome.

It’s important to be aware of what to do and what to avoid while pregnant in order to take care of oneself and one’s developing baby.

What to do

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

  • Take prenatal vitamins.
  • Practice regular physical activity.
  • Work out your pelvic floor by doing Kegel exercises.
  • Eat a diet high in fruits, vegetables, low fat forms of protein, and fiber.
  • Drink about 8 to 10 glasses of water per day.
  • Eat enough calories (about 300 calories more than usual).

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
  • changing cat litter, which can carry a parasitic disease called toxoplasmosis
  • unpasteurized milk or other dairy products
  • uncooked deli meats, such as bologna

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 thinking about during the first few months of your pregnancy, so you can prepare for the future.

When to tell your employer

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

Where you want to give birth

You may want to start considering where you would like to deliver your baby. You can choose to deliver at a hospital, birth center, or your own home. You should weigh the pros and cons of each location and discuss them with your doctor or midwife.

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

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

The first trimester of pregnancy brings many changes as the body produces more hormones to support the pregnancy.

It’s important to attend your prenatal appointments with your midwife or obstetrician. They can give you advice on nutrition, exercise, and emotional support throughout your pregnancy.