From the outside, the development of an unborn baby may just look like a swelling stomach, but on the inside a multitude of rapid changes are taking place. Over the course of nine months, a zygote passes through several distinct phases before transforming into a baby.
Conception
Conception actually happens about two weeks after pregnancy begins, when a sperm fertilizes an egg in the fallopian tubes, according to the Mayo Clinic. You may not conceive on the same day you have sex because a sperm can live for up to five days in the cervical fluid, which can actually increase your chances of having a girl. Female sperm tend to live longer than male sperm, according to BeyondFertility.com.
First Trimester
The first trimester of pregnancy lasts 12 weeks, but often feels significantly shorter because most women don't find out they're pregnant for almost a month, according to the Mayo Clinic. A baby begins as a zygote, a one-celled organism with 46 chromosomes, and then begins to rapidly divide and turn into a blastocyst.
The blastocyst moves down the fallopian tube and then attaches itself to the uterine wall, while placenta begins to form, according to the Mayo Clinic. It becomes an embryo in the fifth week and major organs such as the brain, heart and spinal cord begin to form. The heart begins beating in the sixth week, and further organ and feature development continues until it becomes a fetus in the 11th week.
Second Trimester
The second trimester runs from the 13th to the 27th week, according to the Mayo Clinic. The gender should be identifiable during the 14th week, and organ and bone development rapidly increases. The fetus begins storing body fat during the 17th week and by 20 weeks you may be able to feel it moving -- a milestone often referred to as "quickening," according to the Mayo Clinic. By the 24th week the fetus is regularly waking and sleeping, and it can hear the mother's voice, heartbeat and other noises, according to Pregnancy.org. During the course of the second trimester, the fetus will have tripled in size, according to the Mayo Clinic.
Third Trimester
The third trimester starts in week 28 and lasts until birth, which is usually around week 40. Babies born after the 28th week could survive if they have proper medical attention, according to the Mayo Clinic. Rapid weight gain begins, the baby's eyes open and it practices breathing, and the bones are fully developed by the 36th week.
Full-Term Development
At 37 weeks, a baby is considered full-term and could be born at any point, according to the Mayo Clinic. The baby is fully developed at this point and could survive outside of the womb. It will typically turn itself into a head-down position to prepare for birth, and will weigh between 6 lbs 2 oz and 9 lbs 2 oz, and be between 19 and 21 inches long at birth.
