Steps to Birth

Although there are more than 5000 species of mammals in the world, all of the them, large, small, flying, swimming or walking, share similar stages of foetal development. Here are some of the key milestones that we, humans, share with all of our mammalian relatives.

copulation
The journey to a new life must begin here. For some species, like dolphins, bonobo chimpanzees and humans, the transfer of sperm from male to female is something to be enjoyed and celebrated!

sperm races
Thousands of sperm must fight their way through the female genital tract to get to the egg and fertilise it. This is a perilous journey for most of the sperm. They not only have to survive the female immune system, but they also compete with each other to get there.

fertilisation
This is when sperm and egg unite. Sperm and egg each carry half the genetic compliment needed to make up a new individual: once united the embryo is on its way.

cleavage
This is the process by which a fertilised egg cell gives rise to the all the different cell types of the growing animal. It begins soon after fertilisation, but at first the egg divides to form a hollow ball of undefined cell types, called a blastula.

implantation
The blastula is implanted into the wall of the uterus. Here it will be able to grow and develop into a foetus by taking nutrients from the mother through a network of blood vessels called the placenta.

gastrulation
The cells of the blastula undergo a complex rearrangement, committing each cell lineage to a certain fate. This is when cells are assigned by their position to make specific structures and organs in the adult individual. The placenta begins to form at this stage too.

sex determination
The mammalian X and Y sex chromosomes are responsible for deciding the genital anatomy of the growing foetus. The default status is female. Male foetuses are determined by the presence of a Y chromosome. In humans this switch from default state to male happens at about six weeks.

growth
Once the cells are assigned to specific pathways, the placenta has embedded efficiently and sex has been determined, the foetus can develop and grow by cell division. This is like the baking phase after all the ingredients have been selected!

birth
Finally the embryo is born, often as a result of secreted hormones that either directly or indirectly stimulate the mother's uterine muscles and milk production. Of all mammals, human infants are the most helpless at birth. This is because humans have such big heads that if the foetus were to remain in the womb any longer, its head would be too big to pass through the mother's genital tract without seriously damaging her.

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