However, unlike most other mammals, human childbirth is somewhat dangerous. Babies' heads are large, and the mothers pelvis bones are not very wide. Since people walk on two legs, their hips are fairly narrow. This means that birth can be difficult. Rarely, mother or baby may die in childbirth. The number of mothers dying in childbirth is less in the 21st century. This is because of better medication and treatment. In many poor countries the number of mothers dying is higher. Sometimes it is up to 10 times as many as richer countries.