Prepare & Learn > During Pregnancy > What to Expect During the First Trimester
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What to Expect During the First Trimester

Learn what to expect and how to take care of yourself and your baby during your first trimester.

A healthy first trimester is crucial to normal fetal development. Once you have chosen your doctor or midwife, be sure to schedule your first prenatal visit.
During that first visit a complete medical history will be taken, a physical exam is done and certain tests and procedures – also known as first trimester screening -- will be performed  to assess the initial health of you and your unborn baby.
Be prepared to provide information and answer questions regarding your personal and family health history and use this visit as an opportunity to ask any questions or discuss any concerns that you may have about your pregnancy.

Fetal Development - First Trimester

Early in the first trimester the embryo implants itself into the uterine wall and several major developments take place, including:

  • A sac filled with amniotic fluid, called the amniotic sac, surrounds the fetus, protecting the fetus from injury and regulating temperature.
  • An organ called the placenta, which is shaped like a flat cakeis formed and attaches to the uterine wall with tiny projections called villi. Fetal blood vessels grow from the umbilical cord into these villi, exchanging nourishment and waste products with the mother's blood.
  • The umbilical cord,  a rope-like cord, connects the fetus to the placenta, containing two arteries and a vein. The umbilical carries oxygen and nutrients to the fetus and waste products away from the fetus.
During the first eight weeks, your baby is called an embryo. This embryo develops rapidly and by the end of the first trimester becomes a fully formed fetus, weighing approximately 1/2 to one ounce and measuring, on average, three to four inches in length.
During this time  the fetus is most susceptible to damage from substances such as alcohol, drugs, certain medications, and illnesses such as rubella (German measles).

Learn about fetal development benchmarks during the first trimester>>

Changes to Your Body - First Trimester

Some symptoms of pregnancy continue for several weeks or months, while others are only experienced for a short period of time. And it is important to remember that not everyone experiences pregnancy in the same way.

You may notice the following changes:

  • Swollen and tender breasts in preparation for breastfeeding
  • Frequent urination as your uterus is growing and begins to press on your bladder
  • Mood swings, irritability, and other physical symptoms similar to those that occur shortly before each menstrual period
  • Nausea and sometimes vomiting – referred to as “morning sickness” even though it can occur any time of day
  • Constipation, heartburn and/or indigestion
  • Extreme fatigue due to the physical and emotional demands of pregnancy