Pregnancy is the time during which one or more offspring develops inside a woman's womb. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but can also occur through assisted reproductive technology procedures. A pregnancy may end in a live birth, a spontaneous miscarriage , an induced abortion , or a stillbirth. Childbirth typically occurs around 40 weeks from the start of the last menstrual period , a span known as the gestational age. This is just over nine months. Counting by fertilization age, the length is about 38 weeks. Pregnancy is "the presence of an implanted human embryo or fetus in the uterus"; implantation occurs on average 8–9 days after fertilization. An embryo is the term for the developing offspring during the first seven weeks following implantation , after which the term fetus is used until birth. Signs and symptoms of early pregnancy may include missed periods, tender breasts, morning sickness , hunger, and frequent urination. Pregnancy may be confirmed with a pregnancy test. Methods of birth control—or, more accurately, contraception—are used to avoid pregnancy.