Human embryos, computer artwork. Human embryos are an early stage of the development of a human from a fertilised egg cell. This is about the seventh week of development; the limbs are forming and the embryos are beginning to look human. They will be less than 10 centimetres long, and will weigh less than 50 grams. After eight weeks, the developing humans will no longer be called embryos, and will be called foetuses. They will develop for another 30-34 weeks before being born. This array of many embryos could represent human cloning, embryo stem cell research, or the multiple embryos produced in in vitro fertilisation (IVF) treatments.