Gestational dating ultrasound
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.
Basing GA on the LMP tends to result in an overestimation.
For example, with an LMP beginning on June 15, the EDD will be March 22 of the following year. These consist of an outer wheel that has markings for the calendar and an inner, sliding wheel with weeks and days of gestation.
They facilitate the estimation of GA and the calculation of the EDD.
In normal pregnancies, the length of gestation is minimally affected by maternal characteristics.
However, obese women are significantly more likely to go post-term.
The quality of these wheels varies, but in general, the larger wheels yield better results.
More accurate results can be obtained by using computer software or web-based online calculators (eg, as found online at the Gestation Network).
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 perception of fetal movement by the patient, often referred to as quickening, is a relatively late sign of pregnancy, usually occurring at 19-21 weeks' gestation in nulliparous women and 17-19 weeks' in multiparous women.
Before the advent of pregnancy tests and ultrasonography, quickening was often the method by which a suspected pregnancy was confirmed; however, at present, it has little diagnostic value.
In patients who have had ovulation induction, calculate GA from the day of human chorionic gonadotropin administration.
The size of the uterus can be assessed by pelvic examination or by abdominal palpation.
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The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the times at which to perform various screening tests and assessments,such as serum screening, assessment of maturity, and induction of labor for postdate pregnancies.