Babies are usually conceived in the middle of a woman's menstrual cycle, about two weeks after the start of her period. When she goes to the doctor for confirmation of a pregnancy, he'll calculate an estimated due date that's 40 weeks from the beginning of that period. Throughout those 40 weeks of gestation, the fetus grows and develops in a predictable way.
Embryonic Period
According to Medline Plus, the first eight weeks of gestation are known as the embryonic period. The brain, spinal cord, heart and gastrointestinal tract begin to develop in week three and continue to grow and differentiate. Arm and leg buds are visible by the fifth week and keep developing throughout the embryonic period. By the end of the embryonic period, all of the baby's vital organs have started to develop and she has identifiable facial features.
First Trimester
After the end of the embryonic period, the baby enters the fetal period. The embryonic period, plus weeks nine through 12 of the fetal period, make up the first trimester of pregnancy. By the end of the first trimester, the baby is usually about 2.5 inches long and weighs about 1/2 ounce, according to the Mayo Clinic. He has a human profile and functioning nerves, muscles and organs.
Second Trimester
Weeks 13 through 27 are the second trimester, a time of significant growth and development, says the Mayo Clinic. By week 14, the genitalia are developed enough to determine the fetus's sex on ultrasound. The baby will develop a bony skeleton, fat deposits, fingernails and hair. At 20 weeks, she'll weigh about 11 ounces and that weight will rise to nearly 2 pounds by the 26th week.
Third Trimester
Miraculous changes occur in the womb during the third trimester. By week 32, the fetus is practicing breathing and a week later, he's able to detect light, according to the Mayo Clinic. Week 36 is the beginning of rapid weight gain. A week later, the baby is considered full-term with organs capable of supporting life outside the womb. By the 40th week of gestation, babies are usually longer than 14 inches and weigh an average of 7.5 pounds.
Tip
Sutter Health provides an online reference with bullet points and medical drawings showing details of fetal development during each month of gestation.
