Pregnancy is the state of carrying a growing embryo or fetus in the uterus. In mammals, pregnancy is defined as the period between implantation of a fertilized egg (now called a zygote) in wall of the uterus and delivery or other termination. Some people believe that conception, the moment when the sperm and egg first meet in the fallopian tube, is the start of pregnancy, while the legal and medical definition holds that pregnancy begins when the zygote implants in the uterine wall. Human pregnancy lasts about 38 weeks from conception to birth, or 40 weeks from the time of the last menstrual period to birth (a little more than nine months). Pregnancy milestones are usually measured from the date of your last menstrual period and the time since then is called the “gestational age”: e.g. someone who is 10 weeks pregnant is 10 weeks past the first day of their last menstrual period. This can also be referred to as the 11^th week of pregnancy, just as someone who is 10 years old is said to be in their 11^th year of life.