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First trimester

Stages of pregnancy | First trimester | Second trimester | Third trimester

During the first trimester of pregnancy, you are just learning you are pregnant and coming to terms with both your fears and your hopes for the future. You may have been surprised to find out you were pregnant, and you or your partner may have some ambivalent feelings about the pregnancy. You may also begin to have fantasies and dreams about your child.

Many couples find they are less sexually active in the first trimester. As a result of their fluctuating hormones, fatigue and nausea, some, though not all, women find they are less interested in sex.

During these first three months, you will likely also notice the changes that are happening in your body. Most women will experience some of these physical changes during the first trimester:

  • missed menstrual periods
  • breast tenderness
  • nausea and vomiting, also known as “morning sickness”
  • urinary frequency
  • fatigue
  • increased basal temperature
  • weight gain

During the first trimester, the embryo, which began as a tiny cluster of cells, becomes a fetus and develops all of the major organ systems. Here is what is happening:

First month (1-4 weeks)

  • embryo implants (attaches) to the wall of the uterus and develops rapidly
  • heartbeat begins on the 25th day
  • arm buds appear on the 26th day
  • leg buds appear on the 28th day
  • digestive system starts to develop

The embryo is 1/5th of an inch long.

Second month (5-8 weeks)

  • the face is forming
  • spine begins to form
  • brain, liver, kidneys, bloodstream and digestive system are developing
  • arms and legs develop

By the end of the second month, the embryo has become a fetus. It is about 1 and 1/8th inches long and weighs 1/30th of an ounce.

Third month (9-12 weeks)

  • the fetus' eyelids are fused
  • facial features are present, the nose and outer ears form
  • nail beds established
  • movement, such as head turning or sucking, begins
  • teeth buds form
  • all internal organs form
  • swallows amniotic fluid

The fetus is about three inches long and weighs one ounce. The placenta is fully developed and also weighs about an ounce. One to three ounces of amniotic fluid are present. By two months, a rudimentary umbilical cord has begun to circulate blood and nourishment to the fetus. At 12 weeks, the uterus is just above the pubic bone.

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  • A publication of:
  • Women's College Hospital