From conception to birth, a process of about 40 weeks, a baby goes through innumerable phases of development. The process of transforming from a zygote to a baby can be cataloged week by week, but stages of development are marked by three distinct periods: the first, second and third trimester.
First Trimester
The first trimester of pregnancy lasts for 12 weeks, but it can feel much shorter because most women don't discover that they're pregnant for almost a month, according to the Mayo Clinic. The baby goes through many development phases in the first trimester, beginning as a zygote and then becoming a blastocyst, an embryo and finally becoming a fetus just before the second trimester begins. A zygote, a one-celled organism with 46 chromosomes, is formed when sperm fertilizes an egg in the fallopian tube, according to the Mayo Clinic. The zygote becomes a blastocyst when it begins to rapidly divide and travel from the fallopian tube into the uterus, where it attaches to the uterine wall. The blastocyst becomes an embryo during the fifth week, when the brain, spinal cord, heart and other organs begin to form. The embryo goes through more significant development of organs and facial features before it finally becomes a fetus in the 11th week, according to the Mayo Clinic.
Second Trimester
The second trimester of pregnancy stretches from the 13th week to the 27th week, according to the Mayo Clinic. Organ and bone development rapidly continues, and gender should be apparent during the 13th week. The fetus will begin storing fat around the 17th week and will increase in size. The fetus also becomes more aware of its body during the second trimester. It can make slight facial expressions, is beginning to swallow, can hear the mother's voice, heartbeat and other noises, and is also beginning to move, according to Pregnancy.org. Movement usually is felt by the mother around the 20th week, and is sometimes called "quickening."
Third Trimester
The third trimester of pregnancy begins in the 28th week and lasts until the baby is born, usually around the 40th week. A baby born at the beginning of the third trimester could survive with proper medical attention, according to the Mayo Clinic. Organ, bone and system development continues in preparation for life outside the womb, and rapid weight gain begins. A baby will gain about a half pound per week during the last month of pregnancy, according to the Mayo Clinic. A baby in the womb will also continue to explore its new bodily functions, and it can blink, see light and practice breathing, according to Pregnancy.org. A baby is considered full-term at 37 weeks, and may have flipped over into a head-down position to prepare for birth.
