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During the first trimester, babies grow quickly - passing through two official stages of development.

For the first seven weeks, a developing baby is referred to as an "embryo." From eight weeks to birth, the formal term is "fetus."

Here's how the growth progresses:

About a week after conception, between days seven and 10, the fertilized ovum attaches to the lining of the uterus and the placenta starts to form.  By about two weeks, when you miss your first period, the embryo becomes a layered disc on the uterine wall.  As shown below, by about four weeks, major body systems are starting to develop.

Time	Events
4 weeks	Image 4 weeks The embryo’s eyes, ears, nose, spine, digestive tract and nervous system are beginning to form. The tube for the future heart starts beating.
8 weeks	Image 8 weeks The fetus now has all the organs that a full term baby will have. The heart is functioning. Bones begin to form.
12 weeks	Image 12 weeks Tooth buds are present. Fingernails and toenails are forming. Immature kidneys secrete urine to the bladder. External genitalia are forming. The fetus can now move in the amniotic fluid, but you can’t feel it. Your healthcare provider may be able to hear the fetal heart rate with an electronic listening device.

The first trimester: a critical time for healthy choices

The first three months of pregnancy – the first trimester - is a critical time. Your baby is growing and developing rapidly, and is most at risk from hazards such as smoking, alcohol, drugs, infections and X rays. By the end of the first trimester, all your baby’s organs will be formed and functioning.

When should I announce my pregnancy?
This is a personal decision. Some women want to share the news right away. Others may prefer not to tell people until the second trimester because the risk of miscarriage is highest in the first trimester. Only you can decide the “right” time for you.

Resources & Links:

Pregnancy