How Long Is a Pregnancy?
The Weeks, Months, and Trimesters in Full-Term Pregnancy Explained
The standard template answer for the length of a pregnancy is nine months; you probably already had that answer ready before you even started reading this article.
Well, the truth is that it’s a little more complicated than that. Most pregnancies are actually closer to a month longer. So why has nine months become the standard countdown, synonymous with the due date? Let’s answer that question (and more!).
So, How Long Is a Pregnancy?
Let’s start with the headline question: how long is the average pregnancy? First of all, doctors don’t really talk in months, it’s not precise enough (only February is 4 weeks long, for example). Instead, it’s always weeks and days.
What’s considered a full-term pregnancy is 40 weeks in total (or 280 days), which equates to approximately ten months. Human gestation technically begins from the first day of your last period, and this is why people generally get confused about the length of a pregnancy.
You see, most people only find out they’re pregnant when they miss their period. This is already an entire four weeks into the pregnancy, which is basically a whole month. So the countdown effectively starts quite late, at the point where there are nine months (or 36 weeks) left. Hence the confusion!
You have probably also heard of the term trimester, of which there are three in each pregnancy. The reason we use this timeframe is that doctors associate certain milestones with each one. These are important in the development of your baby.
Of course, not every pregnancy is identical. You may experience a range of distinct conditions during pregnancy, the delivery process can also vary enormously, and the length of a common pregnancy can hover anywhere between 38 and 42 weeks, with no cause for concern for the mother or baby. However, if a baby is born before 37 weeks have passed, it will be considered premature.
Calculating Your Due Date
For those of you who are pregnant (congratulations!), you’re naturally most curious about your own due date, rather than the average. Ideally, you should consult a healthcare professional to work out the most accurate likely due date.
Most professionals will use what’s called the Naegele’s Rule to calculate your due date. This method was established back in the 19th century by an obstetrician by the name of, you guessed it, Franz Naegele. It’s such an effective method that it is still the gold standard today. Here’s how it works:
- First, you work out when you had your last period. It is very important to have accurate information about your usual cycle length.
- From the first day of your last period, add seven days.
- Subtract three months.
- Fast-forward one year.
Pretty simple, right? You can find many online calculators that will do all of the math for you, but the key piece of information is the date of your last period. If you already track your period using an app, you’ll have a simple time working out your probable due date.
But Wait, I Have an Irregular Cycle!
Should you have an irregular cycle, don’t despair. Yes, it’ll be a wee bit more difficult to work out your due date, but the actual length of your pregnancy won’t be affected.
For someone with an irregular cycle, the usual basic calculators will not work quite as well. However, there are other reliable methods available. For instance, a healthcare professional can use an ultrasound to measure the fetus and work out the due date from there.
Is a Due Date Accurate?
In short, not exactly. Whether you are using Naegel’s Rule or tracking the development of the fetus using an ultrasound, the due date will likely be off by several days, if not more. According to a study conducted in Australia, for example, only 5% of births (out of a total of almost 19,000) happened on the expected due date.
The point of working out a due date is not to set a particular day on your calendar, but simply to use it as a guide for the tracking of the healthy development of your baby.
Finally, remember that every pregnancy is unique in its own way, and so are you and your baby! Whilst most pregnancies are around 40 weeks long, this does not mean that births outside of this benchmark length are out of the norm.
