Charting to Estimate Your Due Date (Episode 494)

Full Transcript: Episode 494

Lisa: Today I’m sharing a brand new episode in my FAM Research Series. The topic of the day is using the menstrual cycle—or, in research terms, the last menstrual period—to estimate your due date.

We’ll be talking about due date calculations, and I’ll be sharing a study that looked at how accurate the last menstrual period can be in predicting due dates, particularly for identifying preterm births.

The study is titled A Home Calendar and Recall Method of Last Menstrual Period for Estimating Gestational Age in Rural Bangladesh: A Validation Study. If you want to take a look at the abstract, you can find it at fertilityfriday.com/494.

As someone who has been pregnant four times, I wanted to start by grounding this in personal experience. My first pregnancy ended in an early miscarriage. I had been charting, and I remember that I ovulated about a week later than the “standard” day 14.

During the ultrasound, the doctor said the baby was measuring small. I tried to explain I ovulated later, but he dismissed it. This really highlighted how much weight is placed on the first day of your last menstrual period.

If you’ve been pregnant, you know they ask for that date and use it to calculate everything. And if you ovulate later—or earlier—it can skew everything, because they’re assuming day 14 ovulation.

The thing is, when you miss your period and are two weeks post-ovulation, you’re already considered four weeks pregnant, even though conception only happened two weeks earlier.

This matters because decisions are made based on those dates. If your doctor thinks you’re further along than you are, that might affect the interpretation of ultrasound results, like in my case.

When you chart your cycle, especially using cervical mucus and basal body temperature, you can get a very accurate sense of when ovulation happened—within about a day. That’s a powerful piece of information.

In my case, all three of my full-term pregnancies ended in delivery at exactly 38 weeks. That was based on ovulation, not just counting from the last period.

The study I’m sharing today looked at how accurately women could recall the first day of their last period, and how well that estimate compared to early ultrasound dating. And even though they didn’t track ovulation, there was strong agreement between the two.

The researchers took steps to improve accuracy—they visited participants every five weeks, gave them calendars, and asked them to record when their periods started. If they hadn’t recorded it, they asked and wrote it down. If there was no period in five weeks, they did a pregnancy test.

They excluded women who were more than 15 weeks along when the ultrasound was done, because earlier ultrasounds are more reliable for dating. They really tried to make it a fair comparison.

Their goal was to see if last menstrual period dating could be used reliably, especially in low-resource settings where ultrasounds aren’t always available.

And the results showed strong correlation. Even just using LMP, the estimates aligned well with early ultrasounds.

That’s pretty remarkable—especially considering the women weren’t charting ovulation. So imagine how much more accurate it could be if they were.

In my pregnancies, I actually calculated a “fake” last menstrual period by counting 14 days back from ovulation. That way, when doctors asked, I gave them a date that would align with their pregnancy wheel. It avoided a lot of confusion.

And I did that intentionally, because I found that explaining ovulation tracking didn’t really go over well in short appointments. Most providers just wanted the date.

This is another reason why charting is such a powerful tool. It gives you agency and clarity. You’re not left guessing.

Even though this study didn’t include ovulation data, it still supports the idea that the menstrual cycle can be a valid way to estimate due dates. And when ovulation is tracked? That just adds another level of precision.

I think it’s a great example of how fertility awareness can be used practically—not just for avoiding or achieving pregnancy, but for navigating pregnancy care more confidently.

Thanks so much for tuning in. I hope this episode gave you something new to consider—whether you’re currently charting, pregnant, or just learning. Until next time, be well and happy charting.