When sperm is deposited in the vagina, it travels through the cervix and into the Fallopian tubes.
A single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg and half from the father’s sperm. The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells.
The zygote continues to divide, creating an inner group of cells with an outer shell. This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it.
The blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a baby. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream.
The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation. It leads to the various cell types that make up a human being (such as blood cells, kidney cells, and nerve cells).
There is rapid growth, and the baby's main external features begin to take form. It is during this critical period (most of the first trimester) that the growing baby is most susceptible to damage. The following can interfere with the baby's development:
The period of time between conception and birth during which the fetus grows and develops inside the mother's womb is called gestation. In humans, the length of pregnancy, or gestational age, is the time measured from the first day of the woman's last menstrual cycle to the current date. It is measured in weeks.
The time interval of a gestation plus 2 weeks is called the gestation period, and the length of time plus 2 weeks that the baby has spent developing in the womb is called the gestational age. For more information on gestation, see: Gestational age
The following list describes specific changes that occur in the womb: