Probably the most common method of calculating a due date is to use the “last menstrual period” or LMP. Once a woman finds out she is pregnant, an OB or midwife simply asks when the first day of her previous period was. This becomes day 1 of the pregnancy; day 240 is her estimated due date or EDD. (Of course, they have handy wheel for this calculation.)
The biggest problem with using the LMP is the wide variance of cycle norms for women. LMP due date calculation depends absolutely entirely on every woman having a 28-day cycle and ovulating on day 14 of that cycle, without fail, every single cycle of her life.
Please, stop laughing. They do this.
Yes, there are many doctors and midwives who very much believe this method is accurate, or at least simple. It’s certainly a convenient method and it has an air of mathematical certainty.
Unfortunately for them (and us), very few women fit into this category of 28 day cyclers. There are women who seem to have short cycles – less than 28 days. Chances are that these women also ovulate earlier in their cycle, throwing off LMP due date calculation for them. There are women, like myself, who have long cycles and ovulate later than day 14. Personally, I average about 35 days per cycle, and usually ovulate somewhere around day 21. (Side note – if you want to learn about an FDA approved aid to conception that will help you learn your personal cycle norms, check out the fertility awareness method – not to be confused with the rhythm method.)
Then of course there are those of us who have no earthy idea when our LMP was. Maybe we have irregular cycles – 28 days then 45 then 24 then… Maybe we were breastfeeding or otherwise newly postpartum and our usual “regularity” hadn’t yet returned. For those of us with no idea, there is the ultrasound method of determining EDD.
When the doctor asked me my LMP, I told him I didn’t know. Actually it was October 30 (I know because I had one of the worst periods of my life and took two days off work sick as a dog, unable to get out of bed longer than the time it took to clean up every few hours.) I had been using the fertility awareness method and not only did I know my LMP, but I also knew our conception date – the only possible one within a week before my estimated ovulation date. All other “baby-dancing” days were well outside the possibility of fertility. Calculating my EDD based on what I knew about my cycle, I arrived at August 12.
Of course, my OB didn’t know any of this. He didn’t believe charting had any value. When I told him that I didn’t my LMP, he simply scheduled a dating ultrasound early first trimester. Dating ultrasounds require that an ultrasound technician measure different parts of the baby and compare those to established growth charts. Throughout pregnancy, these measurements can vary by at little as millimeters; you can guess how important it is that your ultrasound tech be a really good one.
Our early ultrasound measured my son to be due August 15, a due date that wasn’t humanly possible with the conception date we had. But a later due date is always better than an early one.
Some OBs don’t stop at that early ultrasound or LMP for determining EDD. They continue to change the EDD throughout the pregnancy based on further ultrasounds (and sometimes fundal height, but that’s a whole ‘nother ball game).
Ladies (and gents!), don’t let this happen to you! It is error and is not based on science in the least to continue to change the EDD.
Ultrasound dating, as I have mentioned, depends on technician skill as well as these growth charts. What we tend to forget is that 50% on a growth chart does not mean normal or perfect. It simply means that your child is larger than 49/50% of children, and smaller than 49/50%. 80% means you have a larger than average child, but not necessarily than anything is wrong, just as 30% or even 10% simply means you have a petite child. (The worry stems from not staying around your percentile as you age or suddenly increasing or decreasing as you age.)
At 32 weeks gestation, my son measured around 4 lbs 2 oz, or 80% and up on the growth chart. However, the ultrasound equipment registered this not at 80% for 32 weeks, but at 34 weeks gestation.
Fortunately, my OB didn’t change my EDD based on this late pregnancy ultrasound. I know women who have been in this situation though and did have their EDD changed. Their OBs didn’t account for the possibility their child would be larger (or smaller) than average.
Having a due date change can be a serious problem. Normal pregnancy, as by ACOG definition, can last from 38-42 weeks. Yes 42 weeks. However, in practice, due dates are set in the middle of this range at 40 weeks. You can still have a normal pregnancy and go past your “due date”; OBs’ own organization admits this.
Most women though aren’t “allowed” to do go past their EDD. Once they begin to approach their EDD, even if this was moved from a later estimate, the induction talk may begin. Women are more often than ever finding themselves against going into labor on their own before 40 weeks or facing induction at 40 weeks. (The number of inductions in the US has doubled since 1992.)
My suggestion: A due month (and arguing with your doctor if necessary). I plan on using the due month with my next pregnancy. The due month is a simple concept (I didn’t come up with it and I’m not sure who did). It’s based on that ACOG definition of normal pregnancy – 38 to 42 weeks. A four week period – your due month. For my son, my due date was August 12. I should have just said that he was coming “sometime in August.”
Believe me, using this method, even just with family and friends, is beneficial. Towards the end of a pregnancy, well-meaning family and friends begin to ask how long the doctor is going to “let you go.” As the EDD approaches, the questions change. “Isn’t he going to induce you?” “What’s happening? Is something wrong?”
Save yourself a wee-bit of anxiety (as much as possible at least) and give them just a ball-park, a taste.
“Oh, Junior? He’s coming when he’s ready, sometime end of April or mid-May. No worries.”
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