It takes about 40 weeks from conception for a baby to make its arrival in the world, and during this time many things are happening inside the mother's womb to make it possible for the baby to be born healthy. Even though expecting parents can't see their baby growing and changing during the pregnancy, it's important they know typical babies development week by week in the womb.
Weeks 1 to 6
Conception occurs as the sperm and egg unite and implant in the side of the uterus. Vital organs are being developed during week 5 as the baby's heart, brain and spinal cord begin to form. The size of the baby is about the size of the tip of a pen, and small buds are forming that will become legs and arms.
Weeks 7 to 12
During week 7 the baby's head begins to form as eyelids grow and nostrils become visible. The buds for the arms and legs continue to grow, and by weeks 8 and 9 fingers and toes begin to form. Around week 10, a baby's genitals begin to form, although it is much too early for even skilled doctors and nurses to determine the sex. The 12th week brings the end of the first trimester, when the baby is approximately 2 inches long and weighs only one-third of an ounce.
Weeks 13 to 20
The baby begins making waste in the form of urine at the beginning of the second trimester during week 13. A baby's sex can be determined by week 14 even though the baby is only 3 1/2 inches long and weighs only 1 14.2gr. Around week 17, fat begins to develop under the skin, and in the next week the baby can begin to hear noises. By week 20, a mom may be able to feel the baby move.
Weeks 21 to 27
During the latter half of the second trimester, the baby's eyebrows grow and become visible around week 22 as the baby reaches 1 pound in weight. Taste buds form during week 23 as fingerprints and footprints are created, giving the baby his identity. Hair begins growing on the head during week 24, and the lungs and the immune system grow and mature during the final weeks of the second trimester.
Weeks 28 to 34
Baby begins the third trimester during week 28, when she weighs approximately 1.02 Kilogram and is 10 inches long. By week 29 the bones in the baby's body are fully developed, and from weeks 32 to 34 the baby begins practicing breathing.
Weeks 35 to 40
The final six weeks of a baby's development in the womb are comprised of rapid weight gain as he grows about 1/2 pound each week. All of the baby's organs and extremities are fully developed, making it possible for him to be born healthy, although possibly underweight, if his mother delivers before week 40.
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