Developing Fetus Stages

Weeks One to Four

• The first one or two weeks of a pregnancy actually don't involve the woman being pregnant at all. A pregnancy is typically calculated as beginning on the starting day of a woman's last menstrual period. Conception generally occurs roughly two weeks after the period begins. When sperm and egg combine, the fertilized egg is referred to as a zygote. The zygote floats down the fallopian tube to the uterus for implantation. Around Week 3, the zygote will attach itself to the uterine wall and the placenta will begin to form. At this point the zygote becomes a blastocyst. By the fourth week, the fetus will be considered an embryo. The baby's organs begin to form by the end of the fourth week and it is about 1/25 inch long.

Weeks Five to 12

• Much of a baby's critical development takes place in the remaining of the first trimester. At Week 5 the circulatory system forms and a heartbeat is often discernible on an ultrasound. The respiratory system and digestive system begin to form by Week 6, and facial features begin to take shape. Between weeks seven and nine the umbilical cord forms, fingers and toes form and movement begins in the womb. Neurons, tooth buds and eyelids form during the 10th week, and weeks 11 and 12 see the formation of sex organs, fingernails and toenails.

Weeks 13 to 20

• Weeks 13 to 20 include many physical milestones, and 20 weeks is the halfway point of a typical pregnancy. The baby will usually begin to kick in week 13. Weeks 14 and 15 include hormone development and the formation of skin and pigment. Facial expressions can be seen as early as week 16 and energy-producing fat begins to accumulate in the 17th week. Babies generally begin to hear and produce urine in weeks 18 and 19, and by week 20 most babies weigh a little over half a pound and are about 6 inches long.

Weeks 21 to 27

• Many distinct changes take place in the remainder of the second trimester during weeks 21 through 27. The baby takes some nourishment from the amniotic fluid begins and taste buds develop in weeks 21 and 22. Weeks 23 through 27 include the lungs preparing for life outside the womb, skin becoming less transparent, the formation of fingerprints and a sense of balance developing. By the time the second trimester ends, most babies weigh close to 2 pounds.

Weeks 28 to 34

• The third trimester is filled with developments that prepare the fetus for the birthing process and life in the outside world. By week 28 most babies' eyes will open. Movement intensifies, reproductive and respiratory development continues and the baby's pupils begin to detect light in weeks 29 to 33. By week 30, a fetus usually weighs about 3 pounds and will continue to gain about half a pound a week until birth. A protective coating called the vernix thickens in week 34 and the baby is rapidly growing and putting on weight during this time period.

Weeks 35 to 40

• Rapid growth continues in weeks 35 to 40 as the fetus prepares for birth. By week 36 most babies can latch and suckle effectively and at week 37 a fetus is considered to be full-term. Weeks 38 to 40 see improvements in organ function and the placenta pumps antibodies into the baby to help fight infection during the first months of life. Week 40 includes the due date, and most babies are between 19 and 21 inches long and weigh between 7 and 8 pounds by this point.