Changing our mindset from pregnancy “due dates” to “due windows”

Nervous, but determined, I walked into my OBGYN’s office. It was only my second time there. The first time, I peed into a cup to confirm my at-home positive pregnancy test, filled out an intake form with my last menstrual period (LMP) and average cycle length, and went home. Then, at about 8 or 9 weeks pregnant, I returned with more data. 

My due date was likely wrong. And I was determined to make it right

How an error on my pregnancy intake form almost determined my baby’s due date 

Flashback to my first appointment—I had estimated that my cycle length was about 36 days long. When the nurse walked in to see what I’d written on my intake form, she stopped at that box.  

“You should erase that 36 and write 35 here,” she said, pointing to my estimated cycle length. 

“Oh, okay,” I complied, following her instructions. 

After I went home, this interaction stuck out in my mind. My cycle length was definitely on the higher side of normal. While I was not consistently using a fertility awareness method (FAM) yet, I had faithfully recorded all of the start dates of my menstrual periods for over a year. I pulled up my period tracking app and catalogued 12 months of cycles on a chart. Some were longer than others—in the 40+ day range, while only a couple were close to 28 days. A few “back-of-the napkin” calculations later, and I discovered that my average cycle was 36.8 days long—nearly 37 days. Knowing that this could affect how my due date was calculated—and, in turn, how much I could be pressured to induce labor if I went “overdue”—I made a mental note to correct this data point at my next appointment. 

Now, back at the office for my second visit, I was nervous. What if they didn’t listen to me?  

The midwife who conducted my transvaginal ultrasound walked me through the mechanics of the visit and what we hoped to see of the baby. 

“Do you have any questions?” she concluded. 

“Yes, actually,” I said. I shared with her my average cycle length and asked if we could adjust the due date, based on that information and the ultrasound.  

“Let’s take a look at the ultrasound, but that sounds very reasonable,” the midwife responded. 

There are two ways to estimate your due date—and most OBs and midwives use the less effective one 

Due dates were first developed by German obstetrician Dr. Franz Naegele (1778–1851). His rule starts by calculating from the date of the last menstrual period (LMP), subtracting 3 months, then adding 1 year and 7 days. 

For example, if your LMP was August 1, 2025, you would subtract three months: May 1, 2025. Then, add 1 year and 7 days: May 8, 2026 would be your Estimated Due Date (EDD). For women with regular, 28-day cycles (and assuming that ovulation happens around day 14), this means that calculating 40 weeks from their LMP will reliably produce their EDD. However, as recent research has demonstrated, most women do not have 28 day cycles, and for most women, ovulation does not happen exactly mid-cycle.   

The alternative you need to know about: Prem’s rule

Many OBGYN offices are likely to use Naegele’s rule, but there is another method for calculating due dates: Prem’s rule. Dr. Konald Prem (1920-2015) developed this rule based on women’s post-ovulation temperature rise (for more on this phenomenon, see this article on tracking basal body temperature), which is a more precise data point than LMP in helping to determine when conception most likely happened. 

To apply Prem’s rule, take the first day of your higher post-ovulation temperatures, subtract 7 days, and add 9 months. For example, if your first elevated post-ovulation temperature was on August 1, 2025, you would subtract 7 days: July 25, 2025. Then, you would add 9 months: April 25, 2026 would be your EDD.  

Because Prem’s rule is based on ovulation, and not on menstruation, it establishes an estimated date of conception (and therefore of expected birth) with greater accuracy. It can also apply to women who have lengthy and/or irregular cycles, or women who are breastfeeding and have not yet had their periods return before conception (because, as every woman needs to know, you will ovulate—and therefore be fertile—before you have your first period postpartum!). 

Back in my OBGYN’s office 

I stretched out on the ultrasound bed, my husband holding my hand, as we both watched our black and white little Squish, posing for his first-ever pictures. With the ultrasound complete, a new EDD showed up on the square filmstrip: 6 days later than my original. I was overjoyed. I knew my due date had to be later! 

“We’re not going to change the date,” the midwife said. 

“Why not?” I asked. “Isn’t the ultrasound fairly accurate at 8 or 9 weeks?” 

“It’s policy,” she said. “We don’t change the due date unless it’s 7 days or more than the original date.”  

“Is there any way you can change the policy? 6 days is nearly 7 days,” I asked. 

“I’m sorry, it’s policy. The only thing that will change if you go to 41 weeks is you will switch from midwife care to obstetric care for delivery.” 

I clammed up after that. No more questions. 

On the drive home, I turned the interaction over and over in my mind. I was shocked that a well-respected obstetrics practice would refuse to adjust a due date, even when presented with well-supported evidence. Why would a policy not consider my individualized period data or that early ultrasound? 

Feeling confident that I was likely to go beyond my (incorrect) assigned EDD, and uncomfortable with the outcome of a transfer from midwife care to an obstetrician, I explored other care alternatives in my area. I chose a new midwife that was open to my concerns, who instantly adjusted my due date to the one on the 8-week ultrasound, which I felt better reflected the longer-than-average length of my menstrual cycles. 

Reconsidering due dates 

For hundreds of years, Naegele’s rule has provided the framework for medical professionals to set the approximate gestational age of a baby, and with it, a singular due date. However, a due “date” might not be the correct way to think when your baby “should” be born. In fact, some medical professionals have been questioning for decades how useful it is to provide a singular date for a baby’s arrival, particularly because of the variability in cycle length and ovulation among women. 

In a 1991 medical article titled “Can we abandon Naegele’s rule?” the authors suggest that we should rethink the 40-week due date, offering instead a due window of 38 weeks to 42 weeks [1]. Going even further back, in an article published in 1980 in the South African Medical Journal, the author states “A historical review reveals that Naegele’s rule was based on anecdotal evidence. This rule should be reviewed and calculations of true gestational age should be based on the probable date of ovulation and conception” (emphasis added) [2].

Because Prem’s rule is based on data taken around ovulation (i.e., the rise in BBT), it is far more accurate and, for medical professionals willing to apply it, may help mothers avoid unnecessary early inductions of labor, or anxiety if the baby is going past that 40 week marker. It’s another excellent reminder of the powerful potential of increased body literacy, especially for women of childbearing age. 

The case for the due window

Regardless of which rule is applied, women can be empowered with the understanding that an estimated due date is just that: an estimate. Even the American College of Obstetricians and Gynecologists (ACOG) has clarified that a pregnancy that lasts 41 or 42 weeks is late term (not “overdue”), and only after 42 weeks is a pregnancy considered post-term.

By giving women a due window instead of a singular date, healthcare professionals could alleviate anxiety and provide the necessary space for low-risk mothers to enter into spontaneous labor, rather than birth via induction—which adds unnecessary stress to both baby and mother. 

Much ado about due dates

First and foremost, if you’ve been tracking your temperature, share that data and Prem’s rule with your healthcare professional. Even if you haven’t been tracking your temperature data, knowing your average cycle length (like I did) is a personalized piece of information that pertains to you, and you should find a healthcare professional who will take that into account. 

And, if you’re flying blind in terms of your cycle length (or any other menstrual cycle data), that’s okay!—you can still educate yourself with the facts about due dates above, and use that information to advocate for yourself with your medical team (and you should seriously consider learning to track your menstrual cycles with a fertility awareness method once your cycles return postpartum). 

This article was updated on December 22, 2025 to correct the example calculation of the Naegele rule.

References

[1] Saunders N, Paterson C. Can we abandon Naegele’s rule? Lancet. 1991 Mar 9;337(8741):600-1. doi: 10.1016/0140-6736(91)91653-c. PMID: 1671954.

[2] Dommisse J. Gestational age – fact or fallacy? S Afr Med J. 1980 Sep 13;58(11):449-50. PMID: 6996150.