The developing embryo has a genetic sex from the moment the sperm fuses with the egg, but for the first few weeks of its development it appears to be asexual, with no obviously male or female external or internal characteristics. Even by eight weeks after fertilisation, there is little sign in the soft tissue of a human embryo to indicate whether it will become male or female, but by twelve weeks we start to see evidence of which sex the fetus may be. When Mum is having her first ultrasound scan, it might be possible to determine the sex of the fetus from the visible external genitalia.
