Pregnancy begins when a sperm fertilizes an egg, which then implants into the lining of the mother's uterus. The average pregnancy lasts for approximately 40 weeks, when doctor's measure from the date of the woman's last menstrual period. Babies are considered full term at 37 weeks gestation and those born before that time are considered premature. Premature babies may require hospitalization until the baby is fully developed and can breathe and eat on his own. Babies develop very rapidly in the womb.
Embryonic Period
Babies inside the womb are referred to as embryos during the first eight weeks of pregnancy. A tremendous amount of growth and development occurs during this time period. Net Wellness explains that many of the vital internal organs, such as the lungs, heart and brain, begin to develop during the first month after fertilization. The heart actually begins to beat by the 25th day of life.
Gender
When the egg and sperm join, the gender of the baby is instantly determined. The sperm is responsible for determining whether the baby will be a girl or boy, with the addition of the X or Y chromosome. Ultrasounds to determine gender are typically performed between 18 to 20 weeks gestation.
Skeletal Development
Bones begin to develop during the second month of pregnancy when joints such as ankles and wrists form, as well as the baby's fingers and toes. The bones harden in the eighth month, although the baby's skull remains soft and its bones are able to move and mold to allow for delivery.
Sleep and Wake Cycles
A baby's sleep and wake cycles begin to develop during the fifth month of pregnancy, as noted by the American Congress of Obstetricians and Gynecologists. Expectant mothers will begin to notice the baby is more active during certain times of the day instead of others.
Reproductive Organs
During pregnancy, a baby develops her genitals and reproductive organs during the second month. Baby girls develop all of their eggs during the fifth month, and baby boys' testicles begin to descend into the scrotum.
Weight Gain
Babies gain the majority of their weight during the third and last trimester of pregnancy. In fact, babies gain half a pound each week during the last month of pregnancy. The fat that accumulates underneath the baby's skin will give him his cute baby cheeks at birth.
