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.
Before pregnancy begins, a female oocyte (egg) must be fertilized, by male sperm in a process referred to in medicine as "fertilization," or commonly as "conception". Though pregnancy technically begins at implantation, it is often convenient to date from the first day of a woman's last menstrual period. This is used to calculate the Estimated Date of Delivery (EDD).
Traditionally (according to Naegele's Rule, which is used to calculate the estimated date of delivery (EDD)), a human pregnancy is considered to last approximately 40 weeks (280 days) from the last menstrual period (LMP), or 37 weeks (259 days) from the date of conception. However, a pregnancy is considered to have reached term between 37 and 43 weeks. Babies born before the 37 week mark are considered premature, while babies born after the 43 week mark are considered postmature.
However, the average length of pregnancy depends on various factors. (Caucasian women are more likely to have a longer pregnancy than other women), and a first pregnancy tends to last longer than subsequent pregnancies. For example, a Caucasian woman's first pregnancy lasts an average 274 days from conception (288 days from the last menstrual period).
An accurate date of conception is important, because it is used in calculating the results of various prenatal tests (for example, in the triple screen test). A decision may be made to induce labour if a baby 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. Approximately 3.6% of all women deliver on the due date predicted by LMP, and 4.7% give birth on the day predicted by ultrasound.
In many societies' medical and legal definitions, human pregnancy is arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of fetal development. The first trimester period carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester the development of the fetus can start to be monitored and diagnosed. The third trimester marks the beginning of viability, which means the fetus can survive if an early natural or induced birth occurs. Because of the possible viability of developed fetus, cultural and legal definitions of life often consider a fetus in the third trimester to be a distinct living person.
Morning sickness aflicts about half of all pregnant women, typically only in the first trimester.
Most women feel more energised in this period, and begin to put on weight. The first movement of the fetus, often referred to as "quickening", can be felt, as it begins to form into a recognisable shape.
Final weight gain takes place, and the fetus begins to move regularly. This can be uncomfortable, causing symptoms like weak bladder control and back-ache.
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