Very few women actually deliver on their due date, but due dates still matter. Whether pregnancy came as a surprise or you and your partner have been planning and trying for a baby for months, a positive pregnancy test will yield an important question: How far along am I in my pregnancy? This isn’t a question Google can answer, but your doctor or midwife can help you plan the timing and make sure your baby is growing according to schedule. Turns out, estimating your baby’s due date is quite a science.
Experts say there are multiple ways to calculate how far along you are now in your pregnancy, including early ultrasounds, estimations based on later ultrasounds, measuring fundal height (the distance between your pubic bone and the top of your bump) after 20 weeks gestation, and good old fashioned counting back from your last missed period.
In fact, Dr. Eduardo Hariton, OB-GYN and fertility specialist, says going by the date of the last menstrual period is still the best way to estimate your baby’s due date. “This assumes a cycle length of 28 days, so people with cycles similar to this will have pretty accurate dating. For those with irregular or very different length cycles, however, this will be inaccurate or impossible to calculate,” he says.
Dr. Seth Plancher of Garden City Obstetrics and Gynecology says not many women are lucky enough to enjoy a clockwork cycle. A 2019 study in the journal npj Digital Medicine supports that, finding that only 13% of women have 28-day cycles. For the rest, there are other options for determining how far along you are, from early to late.
1. With An Early Ultrasound
Plancher explains that the absolute best way to calculate how far along you are is to come in for an early ultrasound as soon as you know you're pregnant. Your doctor can compare the data from that ultrasound with your last menstrual period to arrive at your due date.
However, it's important to note that the earlier you receive your ultrasound, the more accurate it will be. Always test for pregnancy after a late period, and if it comes back positive, be sure to make an appointment right away.
Plancher, a strong proponent of ultrasounds performed before the 8th week, says a lot of doctors still don’t do ultrasounds before 8 or 9 weeks. "I like to see people at six weeks because I can get a really accurate crown-rump length, putting you within a few days of the due date every time," Plancher explains. "The earlier you do an ultrasound, the more accurate it is. The later, the more range of error. If I do an ultrasound and they’re within a few days of their last menstrual period (LMP), then I go with the LMP, because that’s probably right. If they don’t have a regular cycle, I go with the early ultrasound."
2. Calculating From Your Last Menstrual Period
Pregnancies are 40 weeks, but you're actually pregnant for only 38 of them. Why? Because for doctors, the pregnancy clock starts its countdown from the day of your last missed period — the tricky date responsible for the infamous two-week wait when trying to conceive.
The absolute best way to date a pregnancy is from the date of conception, but Plancher says that can be hard to track, especially if you have sex often (as you tend to do while trying to conceive).
But the day of a missed menstrual period is also hard to track. I know that my period likes to take her sweet time arriving some months, while on others, she's way ahead of schedule. If you're the lucky lady with a Swiss cycle, I've got great news: you can figure out how far along you are, simply by counting backwards.
"If someone gets their period every 28 days, you can probably get a pretty good due date based on their last period, but that’s rare. I like to confirm their LMP due date with an early ultrasound,” Plancher says.
3. With A Late Ultrasound
An early ultrasound confers all kinds of benefits, according to Plancher. Not only can you get an accurate date on the pregnancy, but you can also check for tubal pregnancies and reduce your risk of miscarriage (hearing the heartbeat does wonders).
Unfortunately, not everyone knows they're pregnant so early. What if you're really caught off guard, like somewhere in the second trimester? "Sometimes pregnant women show up at 15 weeks, and then you have to use an ultrasound — but there’s a wider range of error, and you have to know that," Plancher notes.
Plancher says that first trimester ultrasounds have about a one-week range of error; second trimester ultrasounds have two-week range, and third trimester ultrasounds have an incredible three-week range of error — which is like a month of your pregnancy.
Calculating how far along you are gets harder as gestation progresses because of natural variability in fetal size (how big your baby is) and observer error. Hariton agrees that second- and third-trimester ultrasounds are not ideal for determining due dates. “This is because the normal range of size for the fetus is wider, so you can’t really tell if, for example, this is a small 34-week fetus or a large 30-week fetus,” he says. Reading and measuring those ultrasounds isn't easy, especially as your baby grows and gets to wiggling.
4. Measuring The Size Of Your Uterus
When you measure your uterus in pregnancy, you're measuring your fundal height, or the distance from the pubic bone to the highest point on your belly. Fundal height measurements are most useful after 20 weeks gestation, at which point providers expect the height of your uterus to match the number of weeks you've been pregnant, Mayo Clinic noted. So if you're been pregnant 22 weeks, you should have a fundal height measurement of 22 centimeters. Neat, huh?
Unfortunately, a host of factors can render fundal measurements less accurate. Certified midwife Mallorie Resendez Bassetti says measuring belly size is a common method for estimating due dates, but it’s not very accurate. “Even from the beginning, a doctor or midwife can feel the uterus and estimate how many months along a woman is, but this is not very accurate,” she says. “Measurements could be off if the baby is growing too small, if there is a fibroid in the uterus, or if there are twins.”
Mayo Clinic was careful to note that fundal measurements are generally taken to reassure women and their providers that baby is growing on schedule, and that it's not a particularly accurate method of dating a pregnancy.
5. With An Advanced Pregnancy Test With Weeks Estimator
You've already taken one test, but here's a reason to take another. ClearBlue Easy's Advance Pregnancy Test With Weeks Estimator, for example, could help you figure out when you got so pregnant all of a sudden. However, it will only estimate that you are 1 to 2 weeks, 2 to 3 weeks, or 3 plus weeks pregnant, so you do have to catch it pretty early if you want to learn anything you don't already know.
Remember, if you feel woozy, hungry, fatigued, or have missed your period, you should always take some kind of test — or better yet, visit your doctor.
While catching a pregnancy in the second or third trimester isn't ideal, there's also no reason to feel embarrassed. With all that's at stake, doctors will be more than happy to see you at any point in your pregnancy, and only a doctor deserving of a seriously negative Yelp review will dare to judge. If this happens to you, your doctor will estimate how far along you are in your pregnancy based on an ultrasound and perhaps a fundal height measurement. Of course, it’s always best to have the most accurate due date. “It allows us to properly time tests — genetic tests, ultrasounds, and the glucose screen — and also helps us time birth. Ideally, women go into labor spontaneously and at the right time,” says Resendez Bassetti. “But, if a woman begins to labor prematurely, knowing how far along the baby is determines how we treat it.”
Dr. Eduardo Hariton, M.D., OB-GYN and fertility specialist at the University of California, San Francisco
Dr. Seth Plancher, MD, FACOG, of Garden City OBGYN
Mallorie Resendez Bassetti, CNM, certified nurse midwife
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