Day by Day Development of the Human Fetus
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The First Trimester
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The first week of pregnancy is marked with the woman's menstruation. Although the baby has not yet been fertilized, this is always considered the first week of pregnancy. During the second week of pregnancy, the egg is fertilized by the sperm and becomes a zygote. During week three, the zygote travels from the fallopian tube, where it was fertilized, into the uterus, where it will grow. During week four, the zygote officially becomes an embryo. During week five, the baby's heart starts beating and, in week six, the embryo's basic facial features begin to form as well as the respiratory and digestive systems. In week seven, the umbilical cord forms, and in week eight, the baby's fingers and toes start to form. In week nine, the embryo will begin moving, although it may not be apparent or felt by the mother yet. In week ten, the baby's brain starts producing hundreds of thousands of new neurons per minute. On week eleven, the first signs of the baby's gender become apparent and genitalia begin to form. On week 12, the baby is approximately 3 inches long and has visible fingers and toenails.
The Second Trimester
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During week 13, the baby will begin moving his or her legs in a kicking motion. It may still not be apparent to the mother. In week 14, the baby's prostate, if it is a boy, and ovaries, if it is a girl, begin developing. In week 15, the baby's skin begins to form as well as the hair follicles on the scalp. In week 16, the baby can make a fist and facial expressions. In week 17, fat cells start developing under the skin. In week 18, the baby can hear your heart beating as well as other noises coming through the womb. It is especially important to play music for your baby starting at this time. During week 19, the baby begins producing urine, which goes into the amniotic sac. The baby also develops a layer of vernix over the skin and lanugo, fine hairs on the skin, which protect the baby. During week 20, the baby develops eyebrows and thicker skin underneath the vernix and lanugo. On week 21, the baby's bone marrow starts making blood cells. During week 22, the baby's taste buds form as well as a full supply of eggs in the ovaries if the baby is female. On week 23, air sacs begin filling with air in the lungs, preparing the lungs to breathe air when the baby is born. During week 24, the baby's fingerprints and footprints form as well as regular sleeping patterns within the womb. In week 25, the baby's hands have fully developed. During week 26, the baby's eyes are fully developed although they cannot open yet. At the end of week 27, the baby's length has probably tripled since the end of the first trimester.
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The Third Trimester
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In week 28, the baby's fully formed eyes begin to open. In week 29, the baby will begin kicking more forcefully and the mother will probably feel the movements. In week 30, the baby may start getting hiccups, which the mother will be able to feel. During week 31, the male baby's scrotum will drop from the kidneys into the groin area, and the female baby's clitoris will form completely. During week 32, the lanugo, the fine hair over the baby's skin to protect it, falls off. In week 33, the baby's pupils can dilate and see light. In week 34, the baby's vernix becomes much thicker in order to protect the baby for birth. In week 35, the baby's growth continues rapidly and will continue to roll and slide around--the mother will probably feel these movements. In week 36, the baby will probably bring its head to the bottom of the uterus to prepare for birth. As of the 37th week, the baby is considered to be full-term and is ready to be birthed. In week 40, the baby's due date comes. Do not be alarmed if you give birth a few weeks before or after your baby's due date. The due date is just an approximation of when the baby will be born.
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