All about Due Dates!

Once the excitement of finding out you are pregnant has passed, one of the first things that many pregnant women do is calculate their due date and mark it on the calendar! Most of us don't think about the validity of the date very much. Instead, we just accept that that is the day our baby will arrive. However, it’s important to understand that a due date is more of an estimate than a promise, and the reality is that fewer than 5% of babies are born on their actual due date. In this post, we’ll explore the history behind due dates, how accurate they really are, and what to expect as you near the end of pregnancy.

What Is a Due Date?

A due date is the estimated day when a pregnant person is expected to give birth. It’s typically calculated as 40 weeks (280 days) from the first day of the last menstrual period (LMP). Due dates are essentially the medical system’s best estimate of when a baby might arrive. They are based on averages and assumptions about pregnancy length, but because every pregnancy is unique, the actual delivery can vary widely from the estimated date.

How Are Due Dates Calculated?

Medical professionals use a few different methods to estimate due dates. One way of calculating it from the first day of the last menstrual period (LMP). Adding 280 days or 40 weeks provides the estimated due date. Due dates can also be calculated based upon the date of conception if known (as is the case with conceiving through fertility treatments), or ultrasound measurements which calculate or refine the due date based upon babies size.

The History of Due Dates

The concept of predicting when a baby will be born has a long history. There has always been a long standing desire to be able to predict birth's timing. Ancient cultures and village wise women or midwives used other methods to estimate when a baby might arrive, including noting physical changes in them other, or by tracking the phases of the moon.

Our modern approach to due dates began in the 19th century with a method called Naegele’s Rule. Named after German obstetrician Franz Karl Naegele, this rule remains one of the most widely used formulas for estimating due dates. Naegele’s Rule calculates the due date by taking the first day of the last menstrual period, adding one year, subtracting three months, and then adding seven days. This formula is based on the assumption that a typical pregnancy lasts about 280 days, or 40 weeks.

In the 20th century, the development of ultrasound technology allowed healthcare providers to measure fetal development and adjust due dates based on the size and growth of the baby. However, despite these advances, due dates remain estimates—most babies do not arrive exactly on their calculated due date.

Due dates, while helpful, are essentially the medical system’s best guess for when a baby will arrive. While due dates serve a purpose in helping us plan and prepare for baby, it’s important to remember that only about 5% of babies are born on their actual due date!

How Accurate Are Due Dates?

Is it important know that only around 5% of babies are born on their actual due date. Most babies arrive within a window of about two weeks before or after the estimated date. This means that while a due date helps with planning, it is by no means a guarantee of when your baby will be born!

There are several factors that can have an impact on how accurate your due date is:

The timing of ovulation and conception can vary by as much as a few days! This variability can lead to discrepancies in due date calculations.

Menstrual cycle length varies! The way that we currently calculate due dates assumes a 28 day menstrual cycle with ovulation occurring on day 14. However, many people have shorter or longer cycles, which can mean that their given due date would be incorrect. Women with irregular cycles may have even more variation in their estimated delivery date!

Studies suggest that first-time mothers may go into labor slightly later than those who have had children before. On average, first-time pregnancies tend to last a few days longer than subsequent ones, which can impact the accuracy of due dates for different individuals.

What's the Problem with Putting a Lot of Emphasis on the Due Date?

While there are medical reasons why due dates matter, it's also important to remember that overemphasizing the due date can contribute to, often unwarranted, feelings of stress and anxiety. Some families who expect the baby to be born on the actual due date can begin to feel disappointed, frustrated, or even afraid when it is surpassed, even though it can be very normal for most first time pregnant women to go past dates.

Whether you agree with whether it is warranted or not, many providers begin to bring up the possibility of medical interventions, such as induction, the as the due date approaches. While some of these interventions are necessary, pressure around adhering to the due date can increase the risk of a cascade of interventions that may not align with the families birth plan or preferences, and may ultimately result in a more clinical and less physiological birth experience.

It's also important to touch on the social pressure around the due date! I can speak from experience, when you go overdue, well meaning family members, friends, and even coworkers start calling, texting and emailing to check in. It can make you feel like a bit of a "watched pot" which is stressful. Ultimately, the constant check ins can take you out of your zone and distract you from the deeply personal experience of enjoying the final few weeks of your pregnancy and the start of labour.

What does being "Full Term" Mean and Why is it Important?

A full term pregnancy is generally defined as lasting between 37 to 42 weeks. The classification of full-term is further divided into early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42 weeks and beyond).

In a book I'm currently reading, "In Your Own Time" by Sara Wickham, there's a wonderful quote that I believe perfectly captures the physiological changes occurring in both the baby and mother during the last few weeks of an undisturbed pregnancy.

"In the last weeks of pregnancy, maternal antibodies are passed to the baby - antibodies that will help fight infections in the fist days and weeks of life. The baby gains weight and strength, stores iron, and develops more coordinated sucking and swallowing abilities. His lungs mature, and he stores brown fat that will help him maintain body temperature in the first days and weeks following birth. The maturing baby and the aging placenta trigger a prostaglandin increase that softens the cervix in readiness for effacement and dilation. A rise in estrogen and a decrease in progesterone increase the uterine sensitivity to oxytocin, the baby moves down the pelvis. Contractions in the last weeks may start the effacement and dilation of the cervix. A burst in energy helps pregnant women make final preparations, and insomnia prepares them for the start of round the clock parenting. 

The watchful waiting and the intense wanting of the big day to arrive are all part of nature's plan. When the baby, uterus, placenta, and hormones are ready, labour will start. Additionally, all that preparation sets the stage for an easier labour and a fully mature baby who is physiologically stable and able to breastfeed well right from the start." (Lothian qtd. in Wickham, In Your Own Time pg. )

Why the rush?

Since the publication of the ARRIVE Trial in 2018, many doctors have leaned on its findings to support the offering or recommending of elective induction at 39 weeks, especially for first-time parents. The study suggested that planned induction at this point in pregnancy could slightly lower the chances of a cesarean birth without increasing risks to the baby. As a result, some care providers now hesitate to let pregnancies continue much past the due date, citing these findings as evidence that earlier delivery may be safer or more efficient.

However, the ARRIVE Trial has also sparked debate within the birth community. Critics point out that it only included a narrow group of low-risk, first-time birthing people, making its conclusions less applicable to a broader population. Others note that many people in the “expectant management” group were still induced, which complicates the comparison. There is also concern that the study’s results are sometimes used to justify routine inductions, which can feel more like policy than personalized care. For some, the push to induce can feel rushed — especially when they were hoping for a more physiological, undisturbed start to labor. Ultimately, whether or not to induce is a deeply personal decision, and one that benefits from careful conversation, informed consent, and support.

When Early Delivery Becomes Necessary

It is important to acknowledge that there are times when delivering before your estimated date becomes necessary. Medical conditions such as preeclampsia, intrauterine growth restriction (IUGR), or concerns about the baby’s well-being may lead your care provider to recommend an induction before labor begins on its own. These decisions are never made lightly and are always in the interest of keeping both you and your baby safe.

As doulas, we’re here to support you through these pivots. If your birth plan needs to change, we’ll help you ask thoughtful questions about your options, understand the information being shared with you, and process any emotions that come up along the way. Our goal is to ensure you feel informed, respected, and deeply supported—no matter when or how your baby is born.

Taking the Pressure Off

There are a few tips we, as doulas, can offer to take away some of the pressure surrounding the due date.

1. Choose a Provider Who Aligns With Your Vision. During prenatal visits, don’t be afraid to ask how they typically handle pregnancies that go past 40 weeks. What’s their comfort level with waiting? How do they view due dates — as a general estimate or a firm deadline?  Have this conversation early on, so that you have time to adapt to the information they provide. A provider who sees due dates as part of a broader window and trusts the physiological process of birth can make a big difference in how supported and respected you feel, especially if your baby takes their time.

   
   

2. Be vague about the date! Reduce social pressure and the constant checking in of well meaning family and friends by not sharing your actual due date with them! Instead, refer to the period around your due date as your "due window" or "due month." When people ask you your due date, answer with "Sometime in June!"
   

3. Trust the timing! Remind yourself that babies are not late or early — they’re on their own perfect timeline. Full term is considered anywhere from 37 to 42 weeks, and most babies arrive within that range. It can help to shift the mindset away from one specific day and toward trusting your body and baby to choose the right time. Affirmations like “My baby knows when to be born” can be a supportive daily mantra.

   
   

4. Distract yourself! If you find that you are getting antsy as you approach your due date, plan a fun activity during your due window to help distract you! I personally went out to see theatre, visited the nature museum, and went out to lunch with family and friends. Other ideas include getting a prenatal massage, enjoying a cozy movie night, or playing board games with friends. Distraction can be so helpful in preventing you from over focusing.
   

5. Keep the Nest Cozy. The final weeks and days of pregnancy are alllll about the oxytocin. To avoid stressing about your due date, use this time to lean into rest and nesting. Whether it's snuggling with your spouse, reading your favourite book, washing baby clothes or meal prepping, enjoying your "nest" can be a great way to feel like you are gently preparing and connecting with yourself and your partner rather than waiting.

Conclusion

It's important to remember that due dates are an estimate and not a guarantee. The exact timing of your baby’s arrival can vary, and that’s completely normal. Trusting your body’s natural rhythm, staying informed, asking questions about your options, and surrounding yourself with a supportive team who respects your birth preferences can make a huge difference as you approach the end of your pregnancy. By fostering open conversations with your healthcare provider, you empower yourself to make decisions that align with your vision for birth, no matter when it happens!

A Little Note Before You Go

As doulas, we offer emotional, physical, and informational support throughout pregnancy, birth, and postpartum — but we are not medical professionals and do not provide medical advice, diagnosis, or treatment. The information in this post is intended to spark reflection, support informed decision-making, and encourage meaningful conversations with your primary care provider. Every pregnancy is unique, and we always recommend discussing your specific questions and options with your doctor or midwife, who knows your health history best. We’re here to walk alongside you, ask thoughtful questions, and help you feel supported every step of the way. 💛