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The organs of sexual reproduction are the gonads , which are the ovaries in females and the testes in males. Females produce female gametes, (eggs); males produce male gametes, (sperm).

Fertilisation

When a female is born, each of her ovaries has hundreds of thousands of eggs, but they remain dormant until her first menstrual cycle, which occurs during puberty. At this time, during adolescence, the pituitary gland secretes hormones that stimulate the ovaries to produce female sex hormones, including oestrogen, which helps the female develop into a sexually mature woman. Every 28 days or so from puberty until menopause, between one and three eggs, the size of the head of a pin, are released from the ovaries. A complex set of hormonal events trigger the release of eggs and send them on their way down the fallopian tubes towards the Uterus. This whole process takes about three minutes and is known as ovulation. Sexual reproduction is the fertilization of a female gamete by a male gamete.

Before ovulation, the cervix produces a special mucous for about 3 to 9 days. This mucous is as crucial to fertility as ovulation because without it, the male’s sperm cannot survive nor be transported inside the female to fertilize the eggs. When mucous is present, it nourishes and protects sperm allowing them to live for 3 to 5 days.

Reproduction

Human reproduction is much the same for all mammals. It is the hallmark of life – essential for the cycle of living things. Conception refers to fertilisation of the female egg by the male sperm. When an egg is released from one of the ovaries at ovulation, between day 9 and day 16 of the menstrual cycle, it makes its way down the fallopian tube to the already prepared uterus. During intercourse, the male ejaculates; about one tenth of an ounce of semen is deposited into the vagina. Between 200 and 300 million sperm are in this small amount of semen. If a female and male have sexual intercourse within several days of ovulation, fertilization can occur. The sperm travels through the cervix into the upper part of the uterus and then into the fallopian tubes where the sperm meets the egg. If the sperm manages to enter the egg, it becomes fertilised and a new life can begin. It takes only one sperm to fertilize the egg.

The development of a baby

About a week after the sperm has fertilized the egg, the egg has become a multicelled blastocyst, a pinhead-sized hollow ball with fluid inside, now present in the uterus. The blastocyst burrows into the thickened endometrium, which is the lining of the uterus. It is the Oestrogen that causes the endometrium to thicken and become rich with blood, and progesterone, another hormone released by the ovaries, keeps the thickness of the endometrium constant so that the blastocyst can attach itself securely to the uterus and continue to absorb nutrients from it. This process is called implantation.

The egg grows and develops inside the uterus (womb) over the next 40 weeks under the protection of the newly developed placenta. The egg develops into an embryo and then at eight weeks, it is a foetus with recognisable features such as hands, feet and eyes.

The placenta

The placenta, which is soft and filled with the embryo’s blood, begins to form and grow during the first 4 weeks. The wall of the placenta is very thin which allows the mother’s blood supply to come alongside the foetus but their blood does not mix. Food and oxygen is passed from the mother to the foetus, and the waste and carbon dioxide is passed back from the foetus to the mother by the umbilical cord, which connects the baby to the mother’s placenta. The developing foetus does not breathe inside the uterus, but instead it uses the placenta to get what it needs to develop and survive.

Birth

After about 40 weeks, the baby is ready to be born. Birth is much easier if the head presents itself first into the vagina. There are muscles in the uterus that contract gently to start with and then the contractions get very strong in order to push the baby out of the mother’s body.

The mucus that has formed a plug in the cervix loosens, and with amniotic fluid, comes out through the vagina when the mother's "water" breaks. The early contractions cause the cervix to relax and widen to approximately 10 centimetres, enough for the baby to come through, and can take many hours. The muscles eventually start to push the baby down through the cervix and then through the vagina, which also enlarges slightly to become the birth canal.

The baby is still attached to the uterus by the umbilical cord and the placenta. Once the baby is in the open air, it is able to breath for its self. The placenta is not needed now and is also passed through the vagina. This is called the afterbirth. The umbilical cord is cut and clamped, and as there are no nerves in the cord, it is painless. The mark left is of course, the belly button.