Knowledge is power, they say, and that's never so true as when you're calculating the time you've been pregnant. Whether pregnancy came as a surprise, or your baby was one of mice and men's best laid plans, when the urine test comes back positive, your first thought is likely to be, "How far along am I in my pregnancy?" Due dates matter, and it's important to know that your baby is growing according to schedule.
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.
First Response's website even offers a calculator which combines the date of your last missed period with your average cycle length to approximate a due date. But how many women know their average cycle length? Who even has a cycle anyone could call "average?" Unless you're planning, who's even sure of the date of their last missed period?
According to Dr. Seth Plancher, MD, FACOG, of Garden City Obstetrics and Gynecology, not many women are lucky enough to enjoy a clockwork cycle. For the rest, here are the options for determining how far along you are, from early to late.
1. With An Early Ultrasound
In an interview with Romper, Plancher explained 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.
"A lot of doctors — and I hear about this from other patients — won’t do an ultrasound until eight weeks or nine weeks along, and I think exactly the opposite," says Plancher, a strong proponent of early ultrasounds, which is defined as an ultrasound performed before the 8th week. As early as week six, Plancher is able to measure the fetus from crown to rump, which helps him to date it precisely (a fetus grows about a millimeter per day). Plancher explains:
"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 you last missed period — the tricky date responsible for the infamous two-week wait.
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 arrivng 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. Plancher notes:
"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."
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, somewhere in the second trimester, say? "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.
According to Plancher, 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. 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. According to Mayo Clinic, 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. 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, Mayo Clinic reported, including if you're obese, have fibroids, or are carrying multiples.
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 ClearBlue Easy's 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 can help you figure out when you got so pregnant all of 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, fatigured, 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 a notoriously unreliable late ultrasound and perhaps a fundal height measurement. Because these measures aren't perfectly accurate, you should avoid inductions or scheduled C-sections if possible, to give your baby all the time she needs grow.
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