Once a woman finds out she’s pregnant, after she hears the magical words “It’s a healthy pregnancy so far” the first thing she wants to know is — when is the baby coming? The sheer anticipation and uncertainty are almost palpable, and she simply has to know when she’s due. That’s perfectly understandable for more than just one reason.
Aside from feeling exhilarating joy, pregnant women also need to know their due dates to prepare for both labor and a life with a baby. Being a pregnant woman is no easy task, but being a mother of a newborn is even harder. Therefore, knowing the due date might be comforting as it gives precise information on what we’re preparing for, and, of course, when it’s going to happen.
But how does calculating the due date work?
How do we know for sure when we will get a visit from the stork?
Or, in reality, when do we need to plan for a trip to the emergency room?
Information Needed to Calculate Your Due Date Include:
- Conception date
- The first day of the last period
- Ultrasound results
- IVF transfer date (in case of an IVF conception)
Pregnancy due date calculators use this information to calculate not only the due date but also other data.
For example, you don’t necessarily need to have all the information at hand. If you know the first day of your last period you can easily calculate the due date as well as the conception date.
Furthermore, some of the more in-depth pregnancy due date calculators can also estimate and give a preview of:
- The size and weight of the baby
- Conception/Due date
- Due dates for twins, triplets, or quads
- Keeping track and recommending prenatal testing dates
- Trimester stages
- Development milestones
#1 Calculating Due Date From Your Last Period Date
Taking note of the first day of your last menstrual period (LMP) is crucial for estimating the due date.
Given that the pregnancy lasts 280 days, if we have the first day of our last period, we can use simple addition to calculate the due date as well.
It’s essential to take into account that although most pregnancies last 40 weeks, they’re 38 weeks long — counting from conception.
If this seems overly complicated, there’s another way to use LMP to calculate the due date. Take the first day of your last period, subtract three months, and add seven days to the date you got.
It sounds confusing, but it’s relatively simple. If your last period was on May 5th, subtracting three months gives us February 5th. When we add the seven days, we get the due date — February 12th.
Even though it seems like a complex game of math, this method is often used by doctors to estimate the due date.
However, it’s important to note that delivering on the due date isn’t a given. You can just as easily give birth a week or two before or after your due date.
#2 UsingConception Date to Calculate Due Date
Those of us who don’t have regular cycles can’t reliably use LMP to calculate the due date..
Calculating the due date is easy when you have a period reminiscent of the Japanese trains — always on time. However, if you’re usually late (or early), then LMP isn’t reliable.
Still, you’re all set if you know your conception date. If you add 266 days to your conception date, you’ll get a fairly reasonable due date estimate.
IVF Transfer Date Makes Everything Easier
Of course, sometimes it’s hard to know the exact date of conception. That’s not the case with IVF — therapists performing the operation know the exact moment conception took place with this method of conception. All you need to do is add 266 days to this date and you’ve got the due date.
Knowing the exact conception date is just one of the many advantages of IVF.
#3 When in Doubt — Consult a Scan
Sometimes pregnancy can be a surprise. Thus, we’re unsure about our LMP but also when our last ovulation was, and even when we last had intercourse (date of conception). Not knowing these things makes determining the due date that much harder.
However, a prenatal appointment with an ultrasound scan will ease your worries and give you the necessary information.
Fundal height is a good determining factor for due date. The height from the pelvic bone to the top of the uterus can be measured, and it will closely estimate the due date. The information is all the more accurate when paired with the measurements of the uterine size.
Early ultrasounds that are a reasonably regular practice with women who have high-risk pregnancies can give an estimated conception and due date. What’s more, ultrasounds can also determine pregnancy milestones. By monitoring those, your doctor can quickly assess everything else.
It’s crucial that we know when the baby’s heartbeat became audible and when the first movement was noticed. Given that these happen around 9–10 and 18–22 weeks, both pieces of information are enough to give an educated guess regarding due date.
Due Date — Plan it or Plan Around It?
After conception, you can’t plan your due date. Babies have their own developmental paths and their own “plans.” Therefore, you can plan all you want — the baby is coming out when it feels like it — not the other way around.
However, you can plan your conception date, and, consequently, plan the due date as well. Of course, this can’t be super precise, as we’re talking planning in months, not days.
Can Due Dates Change?
Just because you received a specific date as your due date, that doesn’t mean that it’s set in stone.
Due dates change — nothing is a given and changing due dates aren’t that uncommon.
Furthermore, your due date can change for any number of reasons. Maybe your LMP information was incorrect. Moreover, the ultrasounds could have been off, or your fundal height wasn’t within the normal range — indicating a wrong due date.
Did you know that only 5% of women give birth on their due date?
It’s almost always off by at least a day. Therefore, it’s vital that you remember that your due date is an estimate. It’s not exactly a guess, but it isn’t a guarantee either. So it’s crucial that you don’t get frustrated if your estimated due date was off by a day (or a week) or two.
The best thing to do is to be patient and wait your baby out. It’s going to come out on its own accord and you’ll welcome it with open arms either way.
Summing it All Up: Calculating Due Dates
Figuring out the expected due date is a bit of a science. We’ve been doing it for generations and have gotten really good at estimating the exact date your baby is most likely to be born.
By taking metrics like your last menstrual period (LMP), conception dates we can make an educated guess for the due date. Newer methods also exist, such as measuring key milestones on ultrasound scans can also be used when these other metrics remain unknown.
If you’re not interested in making these calculations yourself, there are online pregnancy calculators you can use by simply entering the information in when prompted.
Of course, these calculations are simply an educated estimate — your baby will come out on their own schedule, even if this means a week or two before or after the expected due date.