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Pregnancy (gestation) is the physiologic process of a developing fetus within the maternal body. Several terms are used to define the developmental stage of human conception and the duration of pregnancy. For obstetric purposes, the gestational age or menstrual age is the time elapsed since the first day of the last normal menstrual period (LNMP), which actually precedes the time of oocyte fertilization. The gestational age is expressed in completed weeks. The start of the gestation (based on the LNMP) is usually 2 weeks before ovulation, assuming a 28-day regular menstrual cycle. The developmental or fetal age is the age of the conception calculated from the time of implantation, which is 4 to 6 days after ovulation is completed. The menstrual gestational age of pregnancy is calculated at 280 days or 40 completed weeks. The estimated due date (EDD) may be estimated by adding 7 days to the first day of the last menstrual period and subtracting 3 months plus 1 year (Naegele's rule).

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The period of gestation can be divided into units consisting of 3 calendar months each or 3 trimesters. The first trimester can be subdivided into the embryonic and fetal periods. The embryonic period starts at the time of fertilization (developmental age) or at 2 through 10 weeks' gestational age. The embryonic period is the stage at which organogenesis occurs and the time period during which the embryo is most sensitive to teratogens. The end of the embryonic period and the beginning of the fetal period occurs 8 weeks after fertilization (developmental age) or 10 weeks after the onset of the last menstrual period.

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Definitions

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The term gravid means “pregnant”; gravida is the total number of pregnancies that a women has had, regardless of the outcome. Parity is the number of births, both before and after 20 weeks' gestation, and comprises 4 components:

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  1. Full-term births

  2. Preterm births: having given birth to an infant (alive or deceased) weighing 500 g or more, or at or beyond 20 completed weeks (based on the first day of the last menstrual period)

  3. Abortions: pregnancies ending before 20 weeks, either induced or spontaneous

  4. Living children

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When gravidity and parity are calculated as part of the obstetric history, multiple births are designated as a single gravid event, and each infant is included as part of the parity total.

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Live birth is the delivery of any infant (regardless of gestational age) that demonstrates evidence of life (eg, a heartbeat, umbilical cord pulsation, voluntary or involuntary movement), independent of whether the umbilical cord has been cut or the placenta detached. An infant is a live-born human from the moment of birth until the completion of 1 year of life (365 days).

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A preterm infant is defined as one born between 20 weeks and 37 completed weeks of gestation (259 days). A term infant is one born between 37 0/7 and ...

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