Dating of pregnancy
Not infrequently, practitioners are called upon to manage pregnancies resulting from in vitro fertilization or intrauterine insemination.The GA of pregnancies resulting from in vitro fertilization can be precisely calculated from the time of embryo replacement; however, conception may be delayed for a few days in pregnancies resulting from intrauterine insemination.The first 2 are subject to considerable error and should only be used when ultrasonography facilities are not available.The date of feeling the first fetal movements (quickening) is far too unreliable to be useful.Knowing the date of the first positive pregnancy test result allows the calculation of a minimum GA. For example, if the test was performed 4 weeks ago and the test is known to return positive results as early as 1 week after conception, then the minimum conceptional age (CA) would be 5 weeks (GA, 5 2 = 7 wk of amenorrhea).
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.
In rare cases, the date of coitus is known, and this may be useful in calculating the length of pregnancy.
For related information, see Medscape's Pregnancy Resource Center.
Therefore, giving a range for the likely date of birth (eg, EDD -2 weeks to EDD 1 week) is more useful.
Infants born before 37 completed weeks' gestation are deemed preterm, whereas those born after 42 weeks' are considered post-term.
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The fallacy in this assumption is that the time of ovulation varies greatly in relation to the menstrual cycle, both from cycle to cycle and individual to individual.