How Your Baby Grows in the First Trimester

From the editors of

Use this timeline of fetal development to track your baby’s growth from conception through the 12th week.

If you’re like many moms, you only found out recently – after your first month – that you were pregnant. Congrats! And welcome to the beginning of a very exciting journey for both your body and your growing baby.

Check out what’s happening inside your expanding uterus. Use this timeline of fetal development to track your baby’s growth from conception through the 12th week. Save this article – and other three in this series on fetal development – to learn how your fetus is growing. Let the countdown begin!


Baby’s Due In…

…40-35 Weeks

Your baby's growth is slow in these early weeks of pregnancy, only measuring about 0.05 millimeters long, but changes are beginning to take place. Your baby's heart, central nervous system, and skeleton are starting to develop.

…34 Weeks

As tiny as your baby is, your developing baby can now be measured from crown to rump, which is from the top of baby's head to its buttocks. Baby measures about 0.08 to 0.16 inches in length at this time.

…33 Weeks

Your baby is growing faster every day and has even doubled in size since last week. Now your baby-to-be measures between 0.16 and 0.2 inches from crown to rump. At the end of the week your baby will grow to about 0.44 inches!

…32 Weeks

By now baby's foundation of organs and structures has been created, but still only measures a mere inch. This is the last week your unborn baby will be called an embryo — next week your baby officially becomes a fetus!

…31 Weeks

The crown-to-rump length of the baby growing inside of you is between 0.9 to 1.2 inches. Your unborn baby is developing rapidly and is now more physically recognizable as a human being.

…30 Weeks

Your baby is growing rapidly and this week marks the point when baby's weight can start to be charted! Your baby-to-be measures almost two inches from crown to rump. Still small, your baby only weighs about 0.18 of an ounce (5 grams).

…29 Weeks

Your baby is growing rapidly. This week your unborn baby weighs about 0.3 of an ounce (8 grams) and measures between 1.75 and 2.4 inches from crown to rump. Baby's head is almost half of his entire body length!

…28 Weeks

Your unborn baby has almost doubled in length in the past three weeks! Now baby weighs between 0.3 and 0.5 of an ounce (8 to 14 grams) and measures close to 2 1/2 inches from head to rump. At this stage, baby's length is a better measure of her overall growth than her weight is. All of baby's organs and body structures are formed by now, but they continue to grow and develop.

…27 Weeks

Your baby now weighs between 0.5 and 0.7 of an ounce (13 to 20g) and measures between 2.6 and 3.1 inches from crown to rump. The head is about half the size of the baby's length. As your unborn baby's eyes move closer together and the ears move to the sides of the head, baby's face is beginning to look more human. External genitalia have developed enough so that baby's gender can now be determined.

…26 Weeks

This week your unborn baby's crown-to-rump length is between 3 and 4 inches and weighs close to 1 ounce (25 grams). Your growing baby's digestive system is now capable of producing contractions that push food through the bowels. Baby is also able to absorb sugar. Your baby's nervous system is more developed and the pituitary gland is beginning to make hormones.

…25 Weeks

At the beginning of your second trimester, your baby-to-be measures about 4 inches and weighs about 1.75 ounces (50 grams). Fine hair covers your baby's entire body and its skin is very thin. Baby's eyes and ears continue to develop. On ultrasound, many babies this old are seen sucking their thumbs. Your baby's bones have already formed and are getting harder and stronger. In fact, you would actually be able to see baby's skeleton on an x-ray.

Here's your chance to put your adorable baby on the cover of American Baby magazine!
•Personalized Pregnancy Calendar
•"I'm Pregnant. Now What?"
•Fetal Development at a Glance
•How Your Baby Grows
•Your Growing Belly

Find your favorite baby names - and the meanings, origins and traits associated with them!

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