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  • Pregnancy is confirmed by the presence of amenorrhea and a positive pregnancy test.

  • It is crucial to diagnose pregnancy as soon as possible to initiate appropriate prenatal care, avoid teratogen (an agent that causes a deleterious fetal effect) exposure, and diagnose nonviable or ectopic pregnancies.

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, gestational age is the time elapsed since the first day of the last menstrual period (LMP), which actually precedes the time of oocyte fertilization. Gestational (menstrual) age is expressed in completed weeks. The beginning of gestation (based on the LMP) is usually 2 weeks prior to 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 duration of pregnancy is calculated at 280 days or 40 completed weeks of gestation. The estimated due date (EDD) may be estimated by adding 7 days to the first day of the LMP and subtracting 3 months plus 1 year (Naegele’s rule).

The period of gestation can be divided into 3 trimesters consisting of 3 calendar months each. 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 LMP.

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. Fetal death is death prior to completed delivery (also gestational age independent); stillbirth is used to describe fetal death at ≥ 20 weeks of gestation. An infant is a live-born human from the moment of birth until the completion of 1 year of life (365 days), and a neonate is an infant within the first 4 weeks (28 days) from birth.

A preterm infant is defined as one born between 20 and 36 6/7 weeks of gestation (259 days). A term infant is one born between 37 0/7 and 41 6/7 weeks of gestation (280 days), and this stage is divided into early term (37 0/7 to 38 6/7 weeks), term (39 0/7 to 40 6/7 weeks), and late term (41 0/7 to 41 6/7 weeks). At term, a fetus usually weighs > 2500 g. Depending ...

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