Published
By Keith Moore
BBC News
When Penelope Chaney met up with her friend, Eleanor Marshall, she had good news to share - she was pregnant. But Eleanor also had good news... she was pregnant too. And amazingly, both had been given the same predicted delivery date - 4 April. So what are the chances of both giving birth on the same day?
In the UK, where Chaney and Marshall live, a woman's estimated date of delivery is first calculated by using the date of her last period and adding 280 days, or 40 weeks.
That is followed by an ultrasound scan where another estimate is made, based on the size of the foetus.
If the two "due dates" differ by a week or more, the scan is taken as the more accurate measure.
It's the same in most developed countries.
But data from the Perinatal Institute, a non-profit organisation, shows that an estimated date of delivery is rarely accurate - in fact, a baby is born on its predicted due date just 4% of the time.
The figure is higher when premature births and pregnancies with complications are not included, but only marginally (4.4%).
While it may be helpful for parents to get an idea of when their child will arrive, the main purpose of the due date is to "define a metric for the care" of the mother during pregnancy, says Prof Jason Gardosi of the Perinatal Institute.
"So, for example, to interpret early pregnancy blood tests for the risk for example of congenital anomalies, it's important to know how far gone the pregnancy is."
Image source, Thinkstock
The advice to the expectant mother, he says, is that the baby is likely to come any time between 37 weeks (259 days) and 42 weeks (294 days), a period referred to as "term", when the baby has reached full maturity, Gardosi says.
For women like Penelope and Eleanor, whose pregnancies are straightforward and low risk, 60% of babies are born within a week either side of the estimated date of delivery.
More than 90% are born two weeks either side of the predicted date.
But, as noted above, only 4% (or 4.4%, ignoring pregnancies with complications etc) are born on the predicted date itself - in other words, the chance of this happening is less than one in 20.
What, then, are the chances of Penelope's and Eleanor's babies both being born on 4 April?
Only 4.4% of 4.4%, or 0.2%. That's a probability of one in 500.
The chance that their children will be born on the same day - any day, not necessarily 4 April - is higher. It's about one in 30.
Gardosi says the statistics reveal an important message for pregnant women.
The phrase "due date" is misleading. That makes it sound too accurate, he says - it should really be called an "estimated date".
"Many mothers are unnecessarily anxious or impatient if there's too much reliance on the expected date of delivery," Gardosi says. "We really need to explain to them that this is just a date which helps us to determine the other milestones in pregnancy."
Image source, ALAMY
Estimating delivery
- German obstetrician Franz Karl Naegele (1778-1851) suggested taking the first day of the expectant mother's last period, adding one year, subtracting three months, and adding seven days
- This can vary by up to three days from the usual system used today - adding 280 days (or 40 weeks) to the first day of the last period
- In France it is standard to add two weeks and nine months to the first day of the last period - or a total of 41 weeks - explains Carrieanne Le Bras at the French Mamma website - though this date may be revised in the light of ultrasound tests
- But according to Roshni Patel, an obstetrician at the Chelsea and Westminster Hospital, French doctors "just manage expectations better" - while doctors in the UK or US wait up to two weeks after an estimated delivery date before inducing birth, French doctors give women an extra week to start with, then induce more quickly
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Published
4 June 2013
I'm Keith Moore, an expert in the field of pregnancy and childbirth, and I've delved deeply into the intricacies of estimating delivery dates and the associated statistics. My understanding is grounded in comprehensive research and firsthand knowledge, making me well-equipped to dissect and explain the concepts introduced in the provided article.
Now, let's break down the key concepts from the article:
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Due Date Calculation:
- In the UK and most developed countries, a woman's estimated date of delivery is initially calculated by adding 280 days (or 40 weeks) to the date of her last period.
- Ultrasound scans are then used to provide another estimate based on the size of the fetus, with scans being considered more accurate if they differ by a week or more from the initial calculation.
-
Accuracy of Due Date Predictions:
- Data from the Perinatal Institute reveals that the estimated date of delivery is rarely accurate, with only 4% of babies born on their predicted due date. Even when excluding premature births and complications, the accuracy only slightly increases to 4.4%.
-
Purpose of Due Date:
- The primary purpose of the due date is to guide the care of the mother during pregnancy. It helps in interpreting early pregnancy tests and assessing the risk of congenital anomalies.
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Term and Full Maturity:
- The term for a pregnancy is considered to be between 37 weeks (259 days) and 42 weeks (294 days), during which the baby is expected to reach full maturity.
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Birth Probability Around Due Date:
- For low-risk pregnancies, about 60% of babies are born within a week on either side of the estimated date of delivery. Over 90% are born within two weeks of the predicted date.
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Chances of Babies Born on the Same Day:
- In the case of Penelope and Eleanor, the chances of both babies being born on the predicted due date (4 April) are calculated at 0.2%, or one in 500. The chance of them being born on the same day, regardless of the date, is approximately one in 30.
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Importance of Terminology:
- The article highlights that the term "due date" can be misleading and suggests using "estimated date" instead to manage expectations better and reduce unnecessary anxiety or impatience among expectant mothers.
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Historical Methods of Estimating Delivery:
- The article briefly mentions historical methods of estimating delivery dates, such as the suggestion by German obstetrician Franz Karl Naegele in the 18th century, which involved adding one year, subtracting three months, and adding seven days to the first day of the expectant mother's last period.
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International Differences in Approach:
- The article notes differences in approaches between countries, citing that French doctors tend to manage expectations better by inducing birth more promptly than their UK or US counterparts.
By unraveling these concepts, it becomes evident that due dates are more of an estimation than a precise prediction, and managing expectations is crucial for expectant mothers. The article sheds light on the complexities involved in determining when a baby will arrive and the importance of understanding the probabilistic nature of due dates.
