Gestation refers to the period of development of the young in the uterus from the time of fertilization of the ovum (oocyte) to the moment of birth. In other words, it refers to the period where a mother is carrying her young inside her uterus. In human females, the average length of gestation is 280 days, which is equivalent to 40 weeks or about 9 months. It is calculated from the first day of the last normal menstrual period to the time of birth. The duration of gestation can vary, but the normal limits for human gestation are generally from 37 weeks (259 days) to 42 weeks (293 days).