A sample of 838 children aged 5–15 in Australia, England, North America and Sweden were interviewed about how they perceived biological and sexual development. One of the six sections covered gestation and the birth process. Responses were scored on a biological realism scale. The English-speaking children's estimates of the gestation period achieved realistic levels by 11 years, with Swedish children generally at 9 years. In perceiving where the baby emerged at birth, several orifices were named with strong support for cloacal theory at 5 and 7 years. English-speaking children tended to nominate the realistic birth exit by 11 years, Swedish children earlier at 9 years. The necessity of birth at about 9 months was explained realistically between 11 and 13 years although the explanations were couched in crude and simplistic terms. Certain sex and cultural differences are discussed.