How babies are made

This is a brief, sort of technical overview of how a sperm and an egg join forces to make a baby.

Read more about when a woman can and can’t get pregnant

For successful fertilisation of an egg, a live sperm meets with and unites to the egg in a healthy, normal fallopian tube. Fertilisation occurs not long after the egg is released from the ovary – known as ovulation – which occurs about two weeks before a woman’s period is due to begin or two weeks after her period has begun. Each woman’s ovulation is different every cycle, so the day is not always very precise.

When we ovulate, our cervical mucous changes to be more kind to sperm – it allows sperm to swim faster, and creates a friendlier pH. Sperm can remain alive in the vagina for about three days after unprotected sex. Sperm, outside of ovulation, don’t last very long, because the vagina is normally quite acidic due to the friendly bacteria that inhabit it.

The fertilised egg (known as a zygote) divides over and over and over on its way to the implantation site – it travels down the rest of the fallopian tube, down into the uterus. It implants in the endometrial cells (not the uterus specifically), buried in the lining that would otherwise become your period if you weren’t pregnant. Some women get what’s known as implantation bleeding – some spotting a few days after ovulation – but implantation can take many days. The egg essentially burrows into the uterine lining, attaching itself and establishing a blood and nutrient supply. While it does this, it is sustained by the endometrial lining.

This dividing of the fertilised egg occurs for about 5-8 days, until implantation is complete, and the egg turns into a layer of cells around a cavity (blastocyst). The wall of this little thing is one cell thick, except for one section, which is the ‘pole’ – this is the part that implants first and becomes the embryo.

After a couple of days post-implantation, a new layer of cells develops around the single layer of cells, which helps to penetrate the endometrium, anchoring the placenta in place, and other cells that produce a special hormone – this is the hormone tested for in pregnancy tests.

New membranes then develop, which will come to form the amniotic sac. Once the sac is formed, after about 10 days, this little thing is considered an embryo. The amniotic sac fills with fluid and expands out with the growing embryo, filling up the uterus by about 12 weeks after conception. The amniotic sac is then the only cavity in the uterus.

Special cells then transform into the placenta, with special little fingerlike projections (villi) that penetrate the uterus. This sets up circulation between the mother and the foetus. The placenta is fully formed at about week 18, but will keep growing, weighing half a kilogram by the time the pregnancy is complete.

In the embryo, things start to happen – the base level of tissues start to form, creating the base from which the foetus will fully develop over the course of the pregnancy. The heart starts to pump plasma around at about day 20, with immature red blood cells appearing the very next day. Then, the umbilical artery and vein appear, connecting the embryo with the placenta via blood vessels.

Organs form between 21 and 57 days after the egg is fertilised (5-10 weeks), but the central nervous system will develop throughout the entire pregnancy.

Eventually, at about 40 weeks, the baby is ready to be born. A woman goes into labour, and if it all goes smoothly, the baby comes out of her vagina and a new human is born.