Several subjective and objective findings are important when determining the estimated date of delivery for a pregnancy. It’s also important to ask if this last menstrual period was a completely normal menstrual period.
The patient may respond that she only had one or two days of spotting and that this was different than her normal periods, and/or that perhaps it came a few days earlier than she had expected.
When the due date is calculated incorrectly because the length of the cycle wasn’t accounted for, then this error is often magnified when the due date is assumed to be incorrect compared to the dating based on an ultrasound and an even more incorrect due date by ultrasound may be used instead.
Ultrasound (US) has an inherent margin of error for calculating the gestational age. This margin of error is likely less than 8% when the ultrasound is performed between 5.5 weeks and 6.2 weeks, though human error in measuring early pregnancies (such as including the yolk sac in the measurement of the fetal pole) can create significant error.
The estimated date of delivery (EDD) is incredibly important in providing quality prenatal care.
Important decisions at the end of the pregnancy regarding the timing of delivery of both normal and high-risk pregnancies hinges upon the early establishment of the EDD.
The most common error made in determining the estimated due date is not taking into account the accuracy of the last menstrual cycle or the length of the menstrual cycle.But, the reality is, most women do not have a 28 day cycle.Some women who do normally have a 28 day cycle may have other reasons for making the LMP inaccurate; for example, if they were taking birth control at the time of conception or just don’t remember the exact date.This may represent implantation bleeding or other bleeding of early pregnancy rather than an actual period.There’s a difference between the last menstrual period (LMP) and the last normal menstrual period (LNMP). It’s also important to know the characteristics of the patient’s menstrual cycle.This is in the 97th%tile, and that patient is told that she needs to be followed with serial ultrasounds because of the size of the baby and that she is likely to have a very large baby (and the physician is biased to more likely perform a cesarean section).