Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female human. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Obstetrics is the medical field that studies and treats pregnant patients. Childbirth usually occurs about 38 weeks from fertilization, i.e. approximately 40 weeks from the start of the last menstruation.
Before pregnancy begins, a female oocyte (egg) must join with a spermatozoon in a process referred to in medicine as "fertilization", or commonly known as "conception". Fertilization usually occurs through the act of sexual intercourse, in which a spermatozoon penetrates and fertilizes an egg. However, the advent of artificial insemination has made it possible for women to become pregnant if pre-existing medical conditions in either the woman or the man make fertilization through sexual intercourse difficult, or if a woman chooses to become pregnant without a male partner. Though pregnancy begins at implantation, it is often convenient to date from the first day of a woman's last menstrual period, sometimes abbreviated "LMP". This is used to calculate the expected date of delivery. Traditionally a human pregnancy is considered to last approximately 40 weeks (280 days) from the LMP, or 38 weeks (266 days) from the date of fertilization. A pregnancy is considered to have reached term between 37 and 43 weeks from the beginning of the last menstruation. Babies born before the 37 week mark are considered premature, while babies born after the 43 week mark are considered postmature. For example, the first pregnancy tends to last longer than subsequent pregnancies. Fewer than 10% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within two weeks. The due date is typically calculated as 40 weeks from the last menstrual period.
The beginning of pregnancy may be detected in a number of ways, including various pregnancy tests which detect hormones generated by the newly-formed placenta. Clinical blood and urine tests can detect pregnancy soon after implantation, which is as early as 6-8 days after fertilization. Home pregnancy tests are personal urine tests, which normally cannot detect a pregnancy until at least 12-15 days after fertilization. Both clinical and home tests can only detect the state of pregnancy, and cannot detect its age. Diagnostic criteria are: Women who have menstrual cycles and are sexually active, a period delayed by a few days or weeks is suggestive of pregnancy; elevated B-hcG to around 100,000 mIU/mL by 10 weeks of gestation.
The early stages of pregnancy are often discovered by using a pregnancy test, as soon as 48 hours after fertilization using sophisticated testing methods, but not until six to twelve days after fertilization using more typical methods. Pregnancy tests typically detect the presence of human chorionic gonadotropin.
Pregnancy is typically broken into three periods, or trimesters, each of about three months. While there are no hard and fast rules, these distinctions are useful in describing the changes that take place over time.
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