Dating of pregnancy
In normal pregnancies, the length of gestation is minimally affected by maternal characteristics.
However, obese women are significantly more likely to go post-term.
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.
Therefore, giving a range for the likely date of birth (eg, EDD -2 weeks to EDD 1 week) is more useful.
Failure to appreciate this may lead to unnecessary maternal anxiety if a pregnancy progresses beyond the EDD.
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).