How is gestational age assigned for coding and billing purposes?
Traditionally, gestational age is determined by the assigned “due date” or EDD (estimated date of delivery)/EDC (estimated date of confinement). The first day of the LMP (last menstrual period) is the first step in establishing the EDD. By convention, the EDD is 280 days after the first day of the LMP. However, the practice of utilizing the LMP to establish EDD can be inaccurate, as this practice assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle. In addition, utilizing LMP for EDD assignment does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation. In cases where the is concern for inaccurate LMP, dating by first available ultrasound is recommended. Once the EDD is finalized, this is used for all future obstetric encounters to establish gestational age at that visit. In coding/billing practice, when we refer to “weeks gestational age”, we are referring to the gestational age based on the assigned (i.e. predetermined, best available) EDD not the AUA (average ultrasound age) or CGA (composite gestational age) obtained that day, which can often be different from the assigned EDD.