Conception is the very beginning of life. Conception occurs when an egg is fertilised by a sperm and all of the genetic material needed to create life is present. Timing sexual intercourse to ovulation gives the best chance for eggs to be fertilised once they are released.
When a sperm and egg have the good fortune to meet in the Fallopian tube after ovulation, fertilisation occurs. The egg can be fertilised from 12 to 24 hours after being released from its follicle so the odds of conception are highest when sperm are already present in the Fallopian tubes. Having intercourse one to three days before ovulation is ideal to facilitate conception. Once the egg is penetrated by the sperm, the outer membrane's proteins change to make it impossible for another sperm to make its way in. The combined genetic material begins to divide and reproduce itself from the moment of conception.
After conception, the body begins to immediately prepare for an eventual pregnancy. The follicle that produced the egg that was fertilised begins to pump progesterone into the body to stimulate and thicken the endometrium: the lining in the uterus where the foetus will develop. The endometrium develops more blood vessels and folds where the approaching fertilised egg can implant.
At the same time the body is preparing a "home" for the future embryo, the fertilised egg is reproducing rapidly in the Fallopian tube. The genetic materials combine and split into many different cells in a process called meiosis. The fertilised egg quickly becomes a zygote as it travels down the Fallopian tube for approximately three days after conception.
When the zygote reaches the uterus, it has divided so many times that it is now a blastocyst. The blastocyst settles into the uterine wall of the endometrium. This implantation process is officially the start of pregnancy. Attached to the endometrium, the blastocyst is officially an embryo and can begin to develop over the next 32 weeks into a foetus.