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. The 38 weeks of gestation is 10 lunar months, i.e. 27.3 days/lunar-month x 10 = 273 days. In the more familiar Gregorian calendar, the 40 weeks dating from the LMP is equivalent to a little more than nine months and six days. This forms the basis of Naegele's rule of approximating the expected date of delivery, although improvements to Naegele's rule have been suggested. 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.
Though these are the averages, the actual length of pregnancy depends on various factors. 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.
An accurate date of fertilization is important, because it is used in calculating the results of various prenatal tests (for example, in the triple test). A decision may be made to induce labor if a fetus is perceived to be overdue. Due dates are only a rough estimate, and the process of accurately dating a pregnancy is complicated by the fact that not all women have 28 day menstrual cycles, or ovulate on the 14th day following their last menstrual period.
The expected date of delivery may also be calculated from sonogram measurement of the fetus. This method is slightly more accurate than methods based on LMP. Confinement, the beginning of labor, begins on the day predicted by LMP 3.6% of the time and on the day predicted by sonography 4.3% of the time.
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Information on this website has been sourced from Wikipedia contributors. Pregnancy. Wikipedia, The Free Encyclopedia. www.wikipedia.org