The human baby spends nine months developing and growing in the mother's womb. However, most of the organs exist after three months and by six months the foetus is complete. It is a foot long and growing rapidly. There is evidence of considerable neural activity an the foetus can respond to light and touch. With medical help, it can survive outside the womb

The earliest stages of development are when most change happens, going from a single cell to an embryo with organs like the heart and the beginnings of a nervous system before many mothers are even aware that they are pregnant.

The earliest cell division stages occur within days of fertilisation with the Blastula implanting in the wall of the uterus approximately five days after fertilisation. The walls odf the hollow blastocyst will go on to form the placenta and the membranes surrounding the embryo, whilst the block of cells attached to the inside wall of the blastula give rise to the embryo itself.

At six weeks of age, the embryo is about 8mm long with a heart and the brain starting to develop.

At eight weeks, the embryo is a little under an inch long (about 20mm). It is beginning to take on a human look with fingers and toes. However, the placenta is still not fully formed and all the organs are rudimentary. Fingers, toes and muscles have developed and the embryo can make very small twitches as the muscles activate randomly.

The 12 week baby is at the end of the first trimester of pregnancy. All the organs and structures are now present although they still have some development to do; the foetus/embryo is about two inches (50mm) long. After 12 weeks he baby is no longer an embryo and is now referred to as a foetus. The 12 week model here could also be referred to as a foetus as the model is designed to span development up to 12 weeks of embryonic age.

Organ development continues until the end of the second trimester at six months. After that the foetus is mostly growing within the womb.

The twins are posed with membranes removed for clarity.

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