While “long AF” might feel like the most accurate answer from the perspective of someone approaching week 41 of their pregnancy, what is the length of an average pregnancy? Do we know for sure?

It’s common knowledge that pregnancy lasts about 9 months, right? But where does the very specific due date that we are generally given by healthcare providers come from? And should it be considered an expiry date that cannot be crossed without placing the baby at risk?

Calculating your EDD

The idea that pregnancy lasts 40 weeks or 280 days from the first day of the last menstrual period (LMP) until birth was proposed by a German obstetrician in the 1800s. To calculate your estimated date of delivery, or EDD, using this rule you need to add 9 months and 7 days to the first day of your last menstrual period. Research has shown that this is a good estimate in general, but more recent reviews suggest that this neat, round number cannot be broadly applied to all pregnancies and that we need to consider various factors that may influence the length of a pregnancy. 

Why your EDD might be off

The rule assumes a regular menstrual cycle of 28 days and that ovulation occurs approximately 2 weeks after the first day of the last period (around day 14). But ovulation can occur earlier or later in the cycle so, unless you are sure about when you ovulated (or know exactly when you had sex that led to conception), this can throw out the estimate a bit. Also, some people have irregular cycles or cycles that are shorter or longer than 28 days, which means ovulation may not occur at 2 weeks / 14 days and implantation of the egg in the uterus may take place earlier or later in the cycle. The time it takes the fertilised egg to travel from the fallopian tube to implant itself in the lining of the uterus (endometrium) can take anywhere from 6 to 12 days! 

Unless you are tracking your cycle closely, you may also be unsure about when your last period was, which will obviously influence the calculation. 

Research shows that “ethnicity, height,…parity [number of times you have given birth] and maternal weight” can also affect the length of pregnancy.

Other ways to estimate EDD

Dating Scan

This calculation is not the only way to estimate your due date. An optional ultrasound scan can also be used to “date” a pregnancy, particularly between 11 and 14 weeks. The accuracy of a dating scan decreases considerably after week 20, when it becomes more reliable to use the last menstrual period, if it is known. The reason scans become less accurate is that the size of babies varies a lot more in late pregnancy than in the first weeks and months. 

In South Africa, especially when under the care of an obstetrician in private practice, it is common to have scans at almost every check-up during pregnancy. While your care provider may use these scans to assess various aspects of your baby’s health, the American College of Obstetricians and Gynecologists, ACOG, recommends that dates should only be changed based on a scan in later pregnancy in rare circumstances.

The accuracy of scans is also influenced by various factors such as skill and experience of the person doing and interpreting the scan, equipment, biological differences and maternal factors (age, parity, pre-pregnancy weight, geographic location, population characteristics).


ACOG EDD Calculator

In 2016 ACOG launched the Estimated Due Date Calculator, which is an app that is based on recommendations from a joint committee including ACOG, the American Institute of Ultrasound in Medicine (AIUM) and the Society for Maternal-Fetal Medicine (SMFM), for determining pregnancy due dates. What makes this app stand out amongst the countless other pregnancy-related apps available is that it “reconciles the discrepancy in due dates between the first ultrasound and the date of the last menstrual period.” It also takes into account assisted reproductive technology (ART) to help provide more accurate dates for people who undergo IVF.

Fundal Height

If you are under the care of a private midwife in South Africa, you may find that they measure your belly with a tape measure at each antenatal appointment. This measurement is called the fundal height and is a tool used to monitor the growth of a baby in relation to it’s gestational age. It is taken from the pubic bone to the top of your uterus, the fundus. After about 24 weeks of pregnancy, this measurement in centimeters is roughly equal to the number of weeks gestation. While not an exact science, the fundal height measurement can be used by your care provider to help identify potential problems with growth or amniotic fluid levels. If there are concerns you may be referred for a scan to identify the reasons for unusual growth or for closer monitoring.


Recent Evidence

High induction rates in recent times make it harder to find accurate evidence on the duration of pregnancy, but a study published in 2013 on the length of human pregnancy found that, “Among natural conceptions where the date of conception (ovulation) is known, the variation in pregnancy length spanned 37 days, even after excluding women with complications or preterm births.

A variation in length of 37 days! So even when measured exactly from ovulation, the length of pregnancies can vary significantly. 

Why does it matter if your due date is accurate or not?

Aside from giving you an idea of when to expect your baby to arrive, the EDD is used to make other assessments and decisions during pregnancy, for example: 

  • the timing of tests, 
  • assessing whether test results are within a normal range, 
  • identifying unusually slow or rapid growth, or 
  • making decisions regarding when it would be safe for a baby to be born when there are medical concerns such as pre-eclampsia. 

It is also common in South Africa, and elsewhere, for this date to be used when offering routine induction at a certain point in pregnancy. If an EDD is not accurate, it may mean a baby is born before their lungs and brains have had a chance to develop fully and prepare them for life outside the womb.

When is a baby “early” or “late”?

When we consider all these variations, it’s clear that due dates truly are estimates that should be used to give us an idea of when to expect our babies to make their appearance. It’s not uncommon, though, for babies to be referred to as “early” when they arrive before that exact date and “late” when they’ve not, if given the chance, arrived by then. But when is a baby really considered early or late? When are they on time?

In the past, when one heard that a baby was born at “term”, it likely meant that the baby was born anywhere between 37 and 42 weeks gestation. More recently, research has shown that outcomes for babies can vary significantly, especially when it comes to respiratory issues, depending on when within this 5 week period they are born. This has led to the use of more specific designations, namely: 

  • early term:  (37 weeks of gestation through 38 weeks + 6 days)
  • full term:  (39 weeks through 40 weeks + 6 days)
  • late term:  (41 weeks through 41 weeks + 6 days), and
  • post-term:  (42 weeks of gestation and beyond)

These distinctions help to identify and emphasise the difference that each week of gestation can make to the health of your baby.

As mentioned previously, high induction rates in recent years make an accurate measure of the length of an average pregnancy tricky to determine, but studies that have tried to control for induction rates suggest that for 1st births, 50% happen by 40 weeks + 5 days and 75% happen by 41 weeks + 2 days. The timing is a little earlier for second births. 

Ultimately it is clear that there are variations of “normal” when it comes to the length of pregnancy and that an EDD is a useful estimate, but not an expiry date. A great deal of development, particularly in the brain and lungs, may be happening in those final weeks of pregnancy. So, in the context of a healthy pregnancy, a baby that is labeled “late” according to their EDD, may simply have some more baking to do.

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