Dating the pregnancy

"Dating" a pregnancy is the process of calculating the estimated date of delivery (EDD). Typically, the EDD is set at 40 weeks after the mother's last menstrual period (LMP). This assumes that all women have a standard 28-day cycle with ovulation on day 14.

In many cases, this assumption isn't valid, and then, the EDD should be determined by other means. Doctors like to use early ultrasounds to date pregnancies, since embryos grow at a very well-defined rate during the first few weeks of pregnancy.

In addition, women can "chart" their cycles and identify their personal ovulation date with a great degree of accuracy. I'm not going to get into a whole explanation of charting here, but if you're interested in learning more, I highly recommend the Fertility Friend site or the book Taking Charge of Your Fertility.

With Littles -- October 2007
My own cycles have always been on the long side, suggesting that I likely ovulate later than day 14. I explained this to the nurse at my first appointment after getting pregnant with Littles. I was not charting, but based on this and some other signs, I suspected that my actual EDD was about six days later than my EDD per LMP.

The nurse got me set up with an ultrasound, at what was 8 weeks 4 days per LMP, but 7 weeks 5 days per my suspicion. Littles measured 8 weeks 0 days -- so, closer to what I suspected based on my cycle history.

But Dr. K said that since Littles measured within a week of my EDD per LMP, she would not adjust my EDD. At the time, I thought this was great! Six fewer days of the miserable first trimester to put up with!

I came to regret it at the end of my pregnancy. My OBs would not let me go past 42 weeks gestation, because the risks to the baby increase past that point. Since my EDD had been set based on my LMP, this meant 42 weeks from my LMP.

I ended up having to be induced. Of course, if my suspected EDD was actually correct, I was really closer to 41 weeks at my induction date. We could have allowed Littles to keep baking for another six days before she got into the increased-risk territory.

As I've explained previously, I ended up having a very positive induction experience anyway. But I still felt a little deprived of the experience of spontaneous labor. I still believe that Littles would have come on her own had she had those extra days that she rightfully deserved.

With Noob -- June 2009
I started charting after my pregnancy with Littles, so when I got pregnant with Noob, I knew that I had ovulated nearly two full weeks later than "expected." This was another reason why I insisted on an early ultrasound (in addition to wanting some peace of mind after my previous miscarriage).

The ultrasound was at 8 weeks 2 days per LMP, but only 6 weeks 5 days per my chart. It came as no surprise to me that Noob measured 6 weeks 3 days. I didn't quibble over those two days, since I knew it gave him two more days to bake at the end!

As it turns out, he arrived two days "early" -- that is, precisely on his EDD per my chart. Of course, had we not accurately dated the pregnancy, he would have been considered 11 days "late." Many doctors would have induced him by that point... wrongly, because he was not at all overdue.

With Baby Q -- December 2011
I charted again, and so I know that I ovulated late again. But my midwife, C, does not routinely induce for being overdue. If I reach 42 weeks, as long as everything is fine with Q and me, we'll keep waiting. This is another reason why I didn't insist on an early ultrasound with this pregnancy: I'm less worried about accurately dating it, because I know there's no induction looming at the end.

(By the way, you might be curious as to why my OBs with Littles felt that it was too risky to continue past 42 weeks, but C is fine with it. I'll address that in another post, at some point.)

Also, unlike many doctors, C trusts my chart. When we went through the initial paperwork yesterday, she asked me not only what my LMP was (as all my previous providers have done), but also if I knew my ovulation date and my actual EDD -- both of which I do know, thanks to my chart. That's what we're using for dating purposes. Neither of my previous providers ever asked me about these two key pieces of data.