Support Through the Ebb and Flow of the Childbearing Year

Due Date Calculator

About Due Dates and Their Limitations

One of the most common questions you'll hear when you're pregnant is, "When are you due?" Most likely, your prenatal care provider gave you a specific day, which you circled on your calendar.

But like most questions about life and about children, the answer is usually more complicated than that.

Our language about due dates reflects our need for things to be on time. Even the baby is considered "early" or "late." We have such a social stigma around being late, but due dates are estimations, not deadlines. But because most due dates (bills, library books) are deadlines, we tend to project that framework onto birth. People who tell their family and friends a due date may have to endure 50 calls a day asking, "is the baby here YET?" for an extra week.

But babies are not library books--no one's going to charge your baby an overdue fine. We'd prefer not to see babies come very early or late, but most babies come within a window of 2-3 weeks on either side of the due date--right on time.

How do they come up with these numbers, anyway?

Most doctors use Naegele's Rule, which is where we get the 40 week due date. Those fun little due date wheels usually follow Naegele's Rule. Problems with Naegele's Rule:

  • Naegele came up with the 40 weeks thing a long time ago without doing any research; he just noticed that it seemed to be about 10 lunar months, which is 280 days.
  • This calculation is based upon a 28-day cycle. Though this is the average, most of us have cycles that tend to be a little shorter or a little longer, which throws off the due date. And if your cycle is a lot shorter or a lot longer, your dates will be off even further.

The Wood's Method, sometimes called Nicol's Rule, is based on research and reflects the following important points:
  • those with cycles longer or shorter than 28 days do not ovulate on the 14th day, which means the whole pregnancy needs to be adjusted forward or back a bit for accuracy
  • first babies come a little later than those following
  • estimates that first babies come on average 41 weeks, 2 days; subsequent babies around 40 weeks, 5 days

All that said, here's a fairly comprehensive, queer-friendly, IVF-inclusive due date calculator. I'm pretty proud as I did most of the coding and math myself!

Our "Best Guess" Due Date Calculator

If you know the day you ovulated, conceived, or transferred an embryo (IVF) please enter that here. Otherwise, enter the first day of your last menstrual cycle:

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If calculating based on the first day of your last cycle, to account for individual variation from the "standard" 28-day cycle, please choose the length of your average cycle:

How many previous births have you had?



Let's see what we come up with...

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