The First Trimester

by Shannon Bartlett

Although no one would guess by looking at you, a miracle has begun inside you. During the first trimester, your baby grows from mere cells to tiny being who looks like a baby, with eyes, nose, mouth, arms and legs that move. Your baby's heart begins beating and other organs form.

See how your wee one changes in the week-by-week view for fetal development, or click to learn more about pregnancy at 1-4 weeks, 5 weeks, 6 weeks, 7 weeks, 8 weeks, 9 weeks, 10 weeks, 11 weeks, 12 weeks, 13 weeks or 14 weeks. If you're a member, you can customize your dates and follow along with the Pregnancy Calendar, too!

This trimester can be exciting, exhausting, thrilling and a bit scary, all rolled into one. Your body is changing, working overtime to produce hormones and build extra blood cells.

It's easy to be overwhelmed by all that's new, but take your time adjusting to the emotional and physical changes. Keep in mind you have nine months ahead to enjoy and prepare.

When Does First Trimester End?

The first trimester starts with the first day of your last menstrual period, and ends roughly 13 or 14 weeks later. Three different methods are used to divide up a pregnancy into the three trimesters. Each methods results in slightly different spans for the trimesters.

The development method uses the developmental stages to divide up a pregnancy. The second trimester begins at 12 weeks and the third at 27 weeks.

  • From LMP to 12 weeks, the embryo develops all the major organs.
  • From 12 weeks to 27 weeks the fetus continues developing . At 27 weeks, your baby is considered viable.
  • From 27 weeks on, your baby gets matures and prepares for delivery.

The gestation method divides the 40 weeks of gestation into three equal stages. Using the gestation method, the second trimester begins at 13 weeks 3 days and the third at 26 weeks 6 days.

The conception method splits the 38 weeks after conception development into the three equal trimesters. The two week before conception become part of the first trimester. With this method, the second trimester begins at 14 weeks 5 days and the third at 27 weeks 3 days.

Your Body

Pregnancy Symptoms: During early pregnancy, you'll join one of two camps. You might begin to experience early pregnancy symptoms like nausea, breast changes and fatigue or you might not be able to tell you're pregnant. Moms from each group go on to birth healthy babies.

Emotional Changes: You can blame some of your see-sawing emotions on pregnancy hormones, but there's more to these fluctuations that just body chemistry. Having a baby -- whether your first or not -- is life changing. Your wide range of feelings about the pregnancy might include joy, worry, excitement, and apprehension.