why do some pregnant women give birth after nine months or even before the nine months?

Question: why do some pregnant women give birth after nine months or even before the nine months?
- Keywords:
  pregnancy
Asked by 336green28 to Mary, George on 28 May 2019.
- Mary Kaniu answered on 28 May 2019:
  
  The normal duration of pregnancy is 40 weeks(about 9 months) but a baby is ready to be born from about 38 weeks and can be born as late as 42 weeks .
  
  Sometimes babies come early even before 38 weeks for different reasons it could be because the mum develops an illness e.g high blood pressure or starts bleeding before the due date of the baby . Sometimes if they are twins they tend to come early before the actual dates

Question: why do some pregnant women give birth after nine months or even before the nine months?