The first trimester is a time of significant development for the fetus. During these 12 weeks, the fetus will go from a zygote (an egg fertilized by a sperm) to a 2 ½ inch long fetus who is developing fingernails.
So what developments happen during these first 12 weeks?
For the first two weeks considered part of the pregnancy, the woman’s body is actually getting ready for the pregnancy. MayoClinic.com explained that when calculating a woman’s due date, they add 40 weeks from the start of her last period.
However, the conception occurs about two weeks after the start of that period. So those first two weeks are included as part of the woman’s pregnancy, even though she was not pregnant yet.
When conception does occur, the sperm fertilizes the egg, so that fertilized egg, called a zygote, has 46 chromosomes — the number of chromosomes that a human being normally has.
The zygote then travels to the uterus through the fallopian tubes. During this time, the zygote is dividing, giving rise to more cells; this ball of cells is called a morula.
The morula continues to divide, becoming the blastocyst. Six to 12 days after the conception, the blastocyst has made it to the uterus, where it will imbed itself into the lining of the uterus.
At week five of the pregnancy, the development of essential organs begin, such as the brain, heart, spinal cord and gastrointestinal tract, according to MedlinePlus. These organs and other parts of the body will form out of the three layers of the embryo, which are the ectoderm, mesoderm and endoderm.
The eyes, inner ear, central nervous system and peripheral nervous system will arise from the ectoderm, while the heart will arise from the mesoderm. The endoderm is the layer from which the lungs and bladder will develop.
As the fetus continues to develop, more features will become visible. For example, during weeks six and seven of the pregnancy, the fetus’ leg and arm buds will be visible, and the ear and eyes will be starting to form.
MayoClinic.com noted that during this time, the fetus’ heart begins pumping blood.