I’ve talked at length about “due dates” with clients, friends, and other birthworkers until I am blue in the face. They do serve a general purpose, a rough guide, but they are not the golden standard of which to judge the health or progress of a pregnancy (like so many health care providers would have some of us think).
Generally what happens when someone thinks or knows that they are pregnant, they make an appointment to visit a doctor or clinic. At this appointment they will be asked what the first day of their last menstrual cycle/period (LMP) was and then the health care worker will use a gestation wheel to calculate what 280 days from that date to end up with the pregnant person’s estimated due date (EDD). Seems pretty simple, pretty cut and dry. So, what is the issue?
The first issue is that the magical number of 280 days post LMP for human gestation is an average. All gestating bodies and the babies they are gestating are unique and progress at their own rate. While it is reasonable to use the EDD based on a person’s LMP as a rough guide for judging pregnancy milestones, it should not be used as an absolute.
The second issue is one that has always bothered me; using the 280 day calculation actually makes most people roughly 2 weeks more pregnant than they really are. Say what? So, the average person with an average menstrual cycle length of 28 days will probably ovulate between days 11 and 14 of their cycle, not on day one!! Conception does not happen until on or slightly after the day of ovulation.
Let’s look at basing things around ovulation. If we look at ovulation plus the dates when sex (or insemination) occurred, we can get a better idea of when conception actually happened. The magical number used for an EDD based on conception is 266 days. This number is more accurate than basing an EDD off of a person’s LMP, especially if: a) dates for ovulation/conception are known and b) a person has a cycle length that is significantly less than or greater than the average of 28 days. However, it is still only an average length of gestation for humans, so again, we have to factor in uniqueness of both gestating person and their baby.
I am going to use my own dates for my current pregnancy to demonstrate how much things can very.
My LMP was Saturday, March 28th, 2015 in the mid-afternoon. I have longer cycles that vary between 33 and 36 days long, but my luteal phase (time between ovulation and beginning of next menstrual cycle) is consistently 14 days. I ovulate between day 19 and 22 of my cycle. If I were to just plug the first day of my LMP into a gestation wheel (280 days after LMP), I would be given an EDD (roughly 40 weeks gestation) of January 2nd, 2016. This would place me at 40 weeks and 5 days gestation today, which is still within the realm of acceptable by even the strictest definition of what “normal length of gestation” is for healthy humans.
During my LMP I ovulated on April 16th, 2015 (probably in the mid-afternoon to early evening). This was on day 20 of my cycle. The date I had sex closest to ovulation was early in the morning on the 16th as well, so the likelihood that conception took place on the 16th is pretty high. It is the date I decided to use for dating this pregnancy. Calculating my EDD based on date of conception (266 days post conception), I would be given the date January 7th, 2016 (roughly 38 weeks gestation). This would place me at 38 weeks gestation today, which is also within the realm of “normal length of gestation”. However, it does point to a significant difference in “weeks gestation”, which most health care workers associate with as weeks post LMP. My 38 vs. the wheel’s 40.5 is almost a 3 weeks difference!
I know, it sounds confusing and it really can be, especially for people who end up going past their “due date” or have higher risk issues during their pregnancy such as diabetes, hypertension, or are carrying multiples. These three things listed are often used as reasons (and rightly so, sometimes) to induce labour. This is why it is very important to be as sure about the dating of a pregnancy as one can be before making decisions that could end up in a premature birth!!
I have not covered all the possibilities here or all of the additional ways to help get a better idea of a more accurate “due date”, but hopefully, I have given you some things to think about and consider.
Also, no, I have not given birth yet. I am sure this baby will know when it is time.