How does pregnancy happen?

Pregnancy happens when sperm enters the vagina, travels through the cervix and uterus to the fallopian tube and meets an egg and the fertilized egg implants on the lining of the uterus.

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Sperm are reproductive cells made in testicles and they leave the penis with semen and other fluids during ejaculation. Millions of sperm come out with each ejaculation, however it only takes 1 sperm cell to fertilize an egg and cause pregnancy.

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Eggs (ova) mature in the ovaries over the course of the menstrual cycle. Each month, one or more mature eggs are released into the fallopian tube. In preparation for pregnancy, as the eggs are released, hormones make the lining of the uterus thick and spongy in preparation for the fertilized egg to implant. The mature egg is released from the ovary about halfway through the menstrual cycle, this is called ovulation. The mature egg travels through the fallopian tube for 12 to 24 hours, slowly moving through waiting for any sperm that are around. 

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When semen is in the vagina, sperm cells can swim up through the cervix and to the fallopian tube to meet the egg. Sperm can live for up to 6 days before they die. Fertilization occurs when the sperm cell joins the egg. The fertilized egg moves down the fallopian tube and grows and divides into more cells and forms a ball that soon after attaches (implants) on the lining of the uterus. It can take 5 to 7 days after fertilization for the fertilized egg, that becomes  a ball of cells, to move to the uterus and then implant. 

When implantation happens, the fertilized egg releases pregnancy hormones to prevent the lining of the uterus from shedding. The embryo (the developing baby) forms from the cells on the inside of the ball of cells. The placenta (provides nutrients and support to the baby) develops from the cells on the outside of the ball.