Conception refers to the fertilization of a female egg by male sperm at the beginning of pregnancy. The beginning of the pregnancy is usually calculated from the start of the last menstruation.
The female eggs develop in the female ovaries from follicles. One follicle becomes dominant and develops to full maturity by suppressing the action of the others. During the act of ovulation the follicle splits and releases the mature egg from the ovary. This usually occurs two weeks prior to the beginning of the female’s next menstruation.
Following the release of the egg, the follicle constructs the corpus luteum which produces two hormones, estrogen and progesterone. If the egg remains unfertilized the corpus luteum will degenerate, but will continue to issue the two hormones if the egg does becomes fertilized and conception occurs.
Following the release of the egg it moves to the fallopian tube where it lies until penetrated by a male sperm which can occur within 24 hours of it being released. At the instant of penetration by the sperm the egg is surrounded by a protein which prevents further penetration by other sperm. It is this moment that the sex of the embryo and its genetic make up is determined.
The zygote (fertilized egg) begins to divide (mitosis) within 24 hours of being fertilized and stays in the fallopian tube for 3 days prior to moving into the uterus. At this stage the zygote consists of a bundle of cells known as a blastocyst which attaches itself to the wall of the uterus. The process of cell division continues throughout this time and will continue as the embryo develops and grows.
After about three weeks the blastocyst develops into discreet bundles of cells and the first of the nerve cells are formed. The other cells will subsequently develop, following the instruction and interaction between individual genes, into all of the other body parts and systems.
From the first instant of egg fertilization up to about 8 weeks the offspring is usually referred to as being an embryo. From this time onward, up until the birth of the baby, it is commonly referred to as a fetus.