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The Process of Baby Development During the First Trimester

By Daniel Kirk ; Updated June 13, 2017

The first 12 weeks of pregnancy, known as the first trimester, abound with changes and growth in your unborn baby. Despite a common misconception, you don't get pregnant the instant semen enters your body. It takes up to two weeks for the process to be completed for you to become pregnant. Once you're pregnant, the development of your soon-to-be-born child is rapid.

Third Week

Inside one of your fallopian tubes, your partner’s sperm and your egg come together to create a zygote. Made of 46 chromosomes--23 from each partner--this single-celled body plays a large role in your child’s sex, characteristics and personality. After being formed, the zygote leaves the fallopian tube and moves to the uterus, while splitting into a number of tightly knit cells, with the innermost cells forming the embryo and the others feeding and protecting the embryo.

Fourth Week

The zygote, which at this stage is termed a blastocyst, makes its way to the uterus. Once there, it buries itself in the uterine wall for sustenance, and the placenta begins to develop to nourish your growing child.

Fifth Week

Though the unborn baby is less than 1/4-inch long, her brain, heart, spinal cord and other vital organs begin to form in the fifth week. The baby’s circulatory system also begins the first stages of development. The fifth week is the beginning of the embryonic period.

Sixth and Seventh Week

Some of your baby's basic facial features are now visible during the sixth and seventh weeks, and his heart begins pumping blood. He will even sprout the beginnings of arms and legs. Nostrils, eye lenses and elongated arms become apparent. The baby is now between 1/4 and 1/3 of an inch long.

Eighth Week

With legs and arms growing longer and fingers beginning to develop, your baby begins to move her 1/2-inch body during the eighth week, though this movement isn't detectable by you. She also develops an upper lip and nose, and her nipples and eyes are visible.

Ninth and Tenth Week

Bones begin to form in your baby’s arms, which now bend at the elbows. Your baby’s toes also start developing, and his eyelids and ears continue to be formed. He may be as long as 3/4 of an inch long. Genitals begin forming, though it's too soon to identify what gender your baby is at this point. In the 10th week, your baby’s head gets rounder, the neck begins to develop and the eyelids close to guard the eyes.

Eleventh and Twelfth Week

At the 11th week, your baby ends the embryonic period and enters the fetus stage. The fetus begins to develop external genitalia, weighs nearly 8 grams, has fused eyelids and low-set ears and is forming red blood cells in the liver. In the final week of the first trimester, your baby grows fingernails, measures approximately 2.5 inches long and weighs about 14 grams.

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