Select Page

Factsheet- Understanding Estimated ‘Due Dates’: Their History, Comparison with Other Animals, and Why They Aren’t Set in Stone

Print Friendly, PDF & Email

Abbie Tomson

Midwife MSc, BSc, Yoga Teacher, Project Lead at All4Birth
@enevlorel @All4Birth


When you find out you’re pregnant, one of the first things you want to know is the “due date” — the estimated day when you might expect to welcome your baby into the world. But where did the concept of a due date come from? How accurate are these predictions, and should we rely heavily on them? This post explores the history of due dates, how they compare to other species, and why clinging too tightly to this specific date might not be beneficial.

Historical Context of Due Dates

The concept of calculating a due date has been around for centuries, though the methods and accuracy have significantly evolved. Historically, the estimation of a due date was far less scientific and more based on observation and guesswork. In ancient times, expectant mothers were guided by changes in the seasons or the phases of the moon.

In the early 19th century, a significant development occurred with the formulation of “Naegele’s Rule.” This rule, named after German obstetrician Franz Karl Naegele, is still in use today. It calculates the due date by adding one year, subtracting three months, and adding seven days to the first day of a woman’s last menstrual period (LMP). This method assumes a 28-day cycle and ovulation occurring on the 14th day. 1

Before the advent of modern medicine and tools like ultrasound, estimating due dates was imprecise. Communities often relied on the knowledge of midwives, who used inherited wisdom, observation of the mother’s symptoms, and sometimes even folklore to predict when the baby would arrive.

Early women scored lines into batons to count their menstrual cycle and to measure the length of pregnancies. Pregnancies last 10 lunar months, and counting the number of moon cycles provides an estimation of when birth will occur. Ten moon cycles take 295 days (42 weeks) to complete. 2

Gestation Periods Across Species 3-7

Interestingly, human pregnancies are unique not just in how they are calculated but also in length. Here’s how human gestation compares with some other mammals:

– Elephants: Have one of the longest gestation periods at about 640 to 660 days.

– Dogs and Cats: Typically gestate for about 58-68 days.

– Cows: Have a gestation period of around 280 days, quite similar to humans.

– Chimpanzees: Pregnant for about 230 days.

These comparisons show that gestation can vary widely across species, influenced by factors like size, metabolic rate, and evolutionary traits.

The Fluidity of Due Dates

Despite the use of Naegele’s Rule and advancements like ultrasound, due dates are notoriously imprecise. They are not deadlines but rather estimated windows of time when labour is likely to occur. It is essential to remember:

– Only about 5% of babies are born on their actual due date.

– Pregnancy can naturally range from 37 to 42 weeks, so the ‘due date’ is just at the centre of this normal distribution.

Research has demonstrated that the average pregnancy does not last 40 weeks, with only 35% of women giving birth during the week of their estimated due date 8

Controversies and Evidence

In recent years, the reliance on specific due dates has come under scrutiny. Critics argue that it can lead to unnecessary interventions such as inductions and cesarean deliveries. Some experts advocate for a shift towards a “due month” — a broader window which can reduce pressure on the mother and healthcare providers and acknowledge the natural variability in pregnancy length.

Evidence suggests that allowing labour to begin naturally (when there are no medical reasons to do otherwise) can be beneficial for both mother and baby, reducing the risk of complications associated with inductions and premature birth.

Research has also found that estimating a date of birth impacts how women perceive their bodies and alters the experience of waiting for birth to begin 8


While due dates provide a useful estimate for expectant parents to prepare for arrival, they should be viewed with flexibility. Pregnancy is a complex process that doesn’t always adhere to calculated predictions. Embracing a range of expected dates can help reduce anxiety and unnecessary medical interventions, making the birth experience as natural and stress-free as possible. As always, consult with your healthcare provider to understand better what timing might be best for you and your baby.

Links to other resources


Brain Health from Birth: Nurturing Brain Development During Pregnancy and the First Year by Rebecca Fett

Real Food for Pregnancy by Lily Nichols

Functional Maternity: Using Functional Medicine and Nutrition to Improve Pregnancy and Childbirth Outcomes by Sarah Thompson

Reclaiming Childbirth As a Rite of Passage by Rachel Reed

film-audioFilm Audio and Apps

Baby Buddy app, created by the Best Beginnings Charity

The Great Birth Rebellion Podcast | Due Dates


GOV.UK Maternity Rights


Baby Buddy App

NHS Pregnancy



  1. Naegele, F. K. (1902). The Practice of Midwifery. G. Steinkopff.
  2. Reed, R. (2012). Reclaiming Childbirth as a Rite of Passage: Weaving Ancient Wisdom with Modern Knowledge. Pinter & Martin
  3. Elephant Memories: Thirteen Years in the Life of an Elephant Family. University of Chicago Press.
  4. Root Kustritz, M. V. (2006). The Dog Breeder’s Guide to Successful Breeding and Health Management. Elsevier Health Sciences.
  5. Hoskins, J. D. (1995). Veterinary Paediatrics: Dogs and Cats from Birth to Six Months. Saunders.
  6. Evans, J. W. (1981). Applied Animal Reproduction. Reston Publishing Company
  7. Goodall, J. (1986). The Chimpanzees of Gombe: Patterns of Behaviour. Belknap Press of Harvard University Press.
  8. Reed, R., Rowe, J., & Barnes, M. (2016). Midwifery practice during birth: Ritual companionship. Women and birth : journal of the Australian College of Midwives29(3), 269–278.


Translate »