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January 07, 2008



Also, the 40-week gestation benchmark is simply not valid for the many women whose menstrual cycles exceed 28 days, meaning that they ovulated later in their cycle (assuming standard luteal phase of 14 days or so). Weeks of gestation are roughly calculated based on last menstrual period and assume a 28-day cycle -- presumably because the period is the most verifiable factor when the woman hasn't routinely tracked other fertility signs.

In short, the 40-week benchmark is predicated on a flawed variable: last menstrual period combined with the standard assumption of a 28-day cycle. Women who do track fertility and know they ovulated far later than Cycle Day 14 often have difficulty persuading their OBs that they really aren't overdue yet.

Perhaps this accounts (in part) for your point about why some women can safely wait later than 42 weeks, and some women cannot? I'm surprised this variable doesn't get more coverage generally in discussions about induction. Toni Weschler points it out in her book, but otherwise it doesn't seem to come up often.


Thanks for pointing that out! I ovulate on day 20/21 which would make me appear to be "overdue" but in reality, I have had both babies a day or two before my due date (meaning the date I calculated as my due date).

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