It’s a question you’ll hear over and over until you’re holding a baby in your arms: “When are you due?” Everyone wants to know that magic date, and you’re probably more than a little curious about how far along you are, too. Your due date may become a beacon (my belly will stop growing in three more weeks, right?) or a deadline (I better paint the nursery ASAP), but it’s important to remember that your “due date” will only ever be an estimate of the week you’ll give birth (the vast majority of women deliver between weeks 37 and 41).
That said, there’s one big reason why an accurate estimate is important: in case of complications. “If a woman goes into labour early, then we really want to know how far along she is, as best as we can tell,” says Carol Scurfield, medical director of the Women’s Health Clinic in Winnipeg. “A week can make a big difference as far as the health of the baby is concerned.”
Do your best to calculate your due date as soon as you discover you’re pregnant. Here’s how to do it.
1. Calculate using your last menstrual period (LMP)
By far, the most common and accurate way to figure out your estimated due date is to take the start date of your last normal period and add 280 days (40 weeks), which is the typical length of a pregnancy. “Make sure that the period you’re tracking was the same number of days as usual and the same amount of discomfort, meaning that it felt like a normal period,” says Scurfield. “Sometimes women bleed after they’re pregnant, but that bleeding is different from a period.”
The formula for calculating how far along you are (or number of weeks pregnant) is simply the number of days since the first day of your last menstrual period (LMP) divided by seven:
Days since start of LMP ÷ 7 = # of weeks pregnant
Wondering how to calculate your number of months pregnant?
Days since start of LMP ÷ 30 = # of months pregnant
Why do I calculate from the first day of my last period, not conception?
“Before we understood the physiology of ovulation—and that women don’t ovulate until a couple of weeks after their period, so they really can’t get pregnant until that time—the only thing that primitive physicians and midwives knew was that from the first day of the last period, it was about 280 days before a woman gave birth,” says Scurfield. “All of the research and medical literature have been based on that formula being used.”
I had sex two weeks ago, but my doctor says that I’m four weeks pregnant. How?
88 things nobody tells you about being pregnantIf you have a standard 28-day period, you will likely ovulate around day 14 and are most fertile around days 12, 13 and 14—when you had sex (or underwent a fertility treatment) to conceive. It takes about nine days after ovulation for implantation to occur, which brings you to around day 23. It takes a few days for your pregnancy hormone levels to be detectable by a pregnancy test, bringing you to around day 28. How far along you are in your pregnancy isn’t calculated from conception or implantation but from the first day of your last normal period. If you had your period four weeks ago, then ovulated and had sex two weeks ago to become pregnant, you’re currently four weeks pregnant.
What if I don’t know the start date of my last period?
If you haven’t been keeping track of your periods or have extremely irregular periods, you have to make your best guess for the first day of your last period and go for a dating ultrasound as early as six to seven weeks to get a better idea of your number of weeks pregnant.
2. Calculate using a dating ultrasound
A dating ultrasound, which measures the length of the fetus from crown to rump, is an accurate way to estimate your due date or how far along you are if completed within the first trimester (the first 13 weeks of pregnancy). “Most women have a pretty good idea of when their last period was, within a day or two,” says Scurfield. “If a woman has no idea, you’ll want to try to get her as early as you can. Once you get past 13 weeks, the variability of the growth is such that an ultrasound can be off by a week or so.”
I know my LMP. Do I need a dating ultrasound?
Nope! You can rely on the due date suggested by your last menstrual period, but your healthcare provider may schedule one anyway. “Your doctor may do it for other reasons, but for figuring out dates, it’s really not necessary if she isn’t concerned and there’s nothing odd happening,” says Scurfield.
3. Calculate using a pregnancy test with weeks indicator
All pregnancy tests are designed to detect the pregnancy hormone human chorionic gonadotropin (hCG) in urine, but some pricier tests offer a “weeks indicator” that promises to tell how far along you are. Unfortunately, these tests are pretty vague, stating only one to two weeks, two to three weeks or three-plus weeks pregnant. In addition, the tests measure from implantation, not from your LMP. A test might say that you’re one to two weeks pregnant but you’re actually four weeks pregnant, according to standard pregnancy calculations.
4. Calculate using milestones
Historically, the first heartbeat and first movement are used to get a sense of how far along you are or the age of the fetus, but these forms of measurement are now considered outdated. “We don’t use them anymore because they’re so inaccurate,” says Scurfield. “They’ll give you a rough idea, within weeks, but nothing precise.”
What are the chances of delivering on my due date?
Just four percent. “The due date isan estimate,” says Scurfield. “There are all sorts of factors that go into when a woman delivers. It may have to do with the size of the fetus or the health of the mother. Some women will have one baby early and the next baby late. It’s nice to be able to predict and plan, but babies come when they’re ready.”
How can I ensure the most accurate due date possible?
Start tracking your periods (including start date, end date and symptoms) before you get pregnant so that you can easily tell how far along you are. You can use a calendar or period tracker app, such as Period Calendar, Flo and Clue, all of which are free on iPhone and Android.
Then, if you get pregnant—whether it’s planned or a surprise—you’ll know the start of your last period and can accurately assess how far along you are. At that point, you can sign up for Today’s Parent’s free week-by-week pregnancy newsletter to learn about your baby’s development, how you’ll be feeling and what to expect.
Free pregnancy apps like The Bump, What to Expect and Pregnancy Tracker include pregnancy calendar calculators to estimate your due date, as well as functions to help track the size of your baby, anticipate pregnancy symptoms and plan for doctor’s appointments and other key dates (like finding out the sex of your baby if you wish to, usually around 18 to 20 weeks).