Emily Oster

3 min Read Emily Oster

Emily Oster

How Accurate is Pregnancy Dating?

Q&A on variation

Emily Oster

3 min Read

Is there any correlation between the length of a woman’s cycle and her baby’s gestational age? I have a hypothesis that women with longer cycles have, on average, babies with a greater gestational age. I will use myself as an example: I have a long cycle (about 35 days), and my babies were both born after 40 weeks. When I found out I was pregnant, my pregnancy was dated at 6 weeks, since it was 6 weeks since my last period. However, due to my long cycle, it was likely only 3 weeks since ovulation, meaning my pregnancy “should have been” dated at 5 weeks. Fast-forward, and when my baby was born at 40 weeks, she was “actually” only 39 weeks old. Is there anything in the data to support or refute this? I ask this question mostly out of curiosity but also because it may help guide some decision-making when a pregnancy is approaching term, e.g. some OBs strongly encourage induction right at 39 weeks or 40 weeks, which may not be accurate for all women.

—Curious about correlations

There are several different ways to “date” a pregnancy. The most traditional and simplest is to date from the first day of the last menstrual period. If that was 4 weeks ago, you’re 4 weeks pregnant; if that was 5 weeks ago, you’re 5 weeks, and so on. This approach effectively assumes that the cycle is 28 days, with ovulation at day 14. This is, however, only an approximation, for two reasons.

The first is that cycle lengths vary (this is your point). Someone with a cycle of 35 days would typically ovulate at day 21 (generally, the post-ovulation period is a more consistent length). This means that at 35 days after your last period, you are the same “amount” pregnant as someone with a 28-day cycle is at 28 days. But you are “counted” as 5 weeks pregnant, and they are counted as 4.

Ultrasound image of pregnancy
Mart Production / Pexels

You are absolutely right that this would lead to errors later. There are other problems with menstrual cycle use in pregnancy dating. People have trouble remembering the date of their last period. There is variation in the timing of ovulation and fertilization within the cycle, so even if the cycle length is 28 days, there is still some error. 

These issues aren’t obviously a huge problem, unless we start making a lot of important decisions based on them — if we are worried about preterm and post-term birth, then accurate dating is more important.  

There are ways to more accurately date, notably ultrasound. Early in pregnancy, ultrasounds can typically date the pregnancy to within a day or two, allowing for (usually) a more accurate prediction. Ultrasound can also come with error, so if we want to be as accurate as possible, a combination of period dating, ultrasound, and knowledge of cycle length might be used. 

I did want to answer your more specific question! The answer is: no, there is no specific evidence on this correlation. The logic definitely suggests it might be there, but with all of the other sources of error (people forget their period, etc.), this isn’t one that the data has focused on.

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Amanda
1 month ago

US is only helpful in this regard if used VERY early in pregnancy, which is unnecessary in other regards and often not the standard (my providers didn’t even do appointments of any kind before 10 weeks.) Later on, US is not a more helpful guide to dating than LMP. No method is perfect. there is a reason LMP is still the standard.

Eporter
Eporter
1 month ago

I also have a longer cycle and later ovulation (that I’ve tracked with the pee sticks) at my “pregnant confirmation” ultrasound my OB thought baby looked a little small, we talked about my late ovulation and she adjusted my weeks to be more accurate to actual development. I had a great experience! We took another ultrasound two weeks later to confirm baby’s size was on track with her prediction

Juno123
1 month ago

I don’t understand why last menstrual period is still used when it’s a.) inaccurate and b.) ultrasounds are part of standard care. I wonder how much it affects the data.

U29obGVtYWNoZXJAZ21haWwuY29t
U29obGVtYWNoZXJAZ21haWwuY29t
1 month ago
Reply to  Juno123
1 month ago

I think it’s probably because most people do not know exactly when they ovulated, but they do know when they last menstruated, so there is a standard yardstick measured for every person. I wish they would be willing to consider additional data, like for me I knew exactly when I ovulated and conceived based on ovulation testing and temperature tracking.

Juno123
1 month ago
Reply to  U29obGVtYWNoZXJAZ21haWwuY29t
1 month ago

I understand not knowing when ovulation occurred, but an ultrasound can give a more accurate date. Why isn’t ultrasound dating the standard that is used?

joanne
joanne
1 month ago
Reply to  Juno123
1 month ago

Ultrasound is used as the standard for dating in the UK. On the NHS, everyone has a dating scan between about 10-13 weeks and the pregnancy is officially dated based on that. Seems weird that it’s different in the US! Especially as it seems (from what I hear) that induction is recommended pretty often in the US therefore a more accurate date for the pregnancy gestation would be helpful to decision making.

Helen
Helen
24 days ago
Reply to  joanne
24 days ago

In the U.S. it’s standard to have a dating ultrasound at 8 weeks. I went in at 7 weeks 6 days but was measuring 8 weeks 5 days. I knew I’d ovulated early (cycle day 12) and that I’d gotten a positive on an at-home pregnancy test on CD22. To my surprise, they *did* adjust my due date to match the ultrasound. Whether they change the date based on ultrasound or LMP probably varies by provider here.

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It has been a busy summer for the team at ParentData. I’d love to take a moment here to celebrate the 400k milestone. As I’ve said before, it’s more important than ever to put good data in the hands of parents.

Share this post with a friend who could use a little more data, and a little less parenting overwhelm.

📷 Me and my oldest, collaborating on “Expecting Better”
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Comment “Link” to subscribe to ParentData with Emily Oster, joined by some excellent guests.

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#parentdata #emilyoster #tickseason #bugbites #bugspray
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