Your first week of pregnancy is generally counted as the first day of your last period. Though you will not be pregnant at this time, it is considered the first week of your fetus. Two weeks after your period is when conception generally occurs. Week three is when fertilization occurs when the sperm and egg fuse together in one of your fallopian tubes to form a zygote. If at this time, more than one egg is released and fertilized, then you may have multiple zygotes. By the fourth week, the zygote will attach itself to the uterine wall to nourish itself and your baby's placenta will begin to form.
The fifth week of pregnancy is when your baby's heart, spinal cord, brain and other organs start to develop. By week six, your baby's heart will start to pump blood and the neural tube along his/her back starts to close. During the seventh week of pregnancy, your baby's face and brain begin to develop rapidly, along with his/her eye lenses and arms.
By eight weeks, your baby's legs and arms and fingers will start to form. His/her upper lip and nose start to develop and your baby's ears, eyes and nipples will become visible. By the end of the first trimester, your baby's neck, head, eyelids and genitals will start to develop.
By week thirteen, your baby will be discharging urine into the amniotic fluid and his/her intestines will have moved into the abdomen. In the next few weeks, your baby's scalp, hair and skeleton will begin to develop. His/her sex will become apparent during this period. In your eighteenth week of pregnancy, your baby may be able to hear things around him/her like the beating of your heart and the blood moving through the umbilical cord. By the end of the second trimester, your baby's hands are fully developed and his/her immune system, liver and lungs start to mature. By week thirty, your baby's eyes are wide open and he/she has formed a considerable amount of hair. His/her bones are also completely developed by this time. In addition, your baby will gain weight rapidly in this trimester, by about week thirty six. By week forty, when your due date arrives, your baby's organs are ready to function on their own and he/she will descend into the head down position.
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