Does anyone recall sitting in math class and just staring at the blackboard thinking, “Nope, I just don’t get how X + Y * A = B.” Sure, you nodded to get Mr. Novick off your back, but in reality, you had no idea how he arrived at that answer. And while algebra and amusing are rarely used in the same sentence, that guy seemed to be doing some funny math! Much the same can be said about how we fertility doctors date (a.k.a. tell how pregnant you are and when you are due) pregnancies conceived with fertility treatments.

Most pregnancies (ART excluded) are dated based on the first day of a woman’s last menstrual period. And although you are technically not pregnant in the first approximately two weeks of the menstrual cycle (a.k.a. the follicular phase), you are growing the egg that will ultimately become half of your baby. Because the majority of menstrual cycles range between 25–35 days, the math usually works out. But when pregnancies are conceived with fertility treatments, the lead time (a.k.a. egg development) can be VERY variable. Weeks and even months may be added to get an embryo implant ready. For this reason, if you used fertility treatments to conceive, you can’t simply add a few days to your last menstrual period to calculate your due date. You may need some creative counting and a good doctor to get things sorted out.

As much as we love numbers, we can only count so high! Therefore, to make sure we are getting this equation right, we use pregnancy wheels (a fancy way to say pregnancy calculators) to figure out when you should plan to meet your plus one. Whether it be the day you ovulated, the day we performed your IUI, or the day your IVF ET was done, we can figure out exactly how far along you are. Given that the numbers will be less than transparent, it’s important to get a due date calculated by your fertility doctor before posting “We are expecting…coming March 24th” on your Facebook page. Additionally, you want to pass this info on to your OB and any other practitioner that participates in your care—they need to be in the know about when you will be ready to go!

Dates are not only numbers to an OB. We don’t break them, we don’t forget them, and we certainly don’t change them (unless we have a really, really good reason). They not only dictate when pregnancy-specific tests should be run (think genetic screening, diabetes screen, and GBS screening) but also when a fetus has what it takes to take on the world. Fetal lungs weren’t built in a day. In fact, they weren’t even built (for most babies) in nine months. They require those dreaded extra four weeks (remember, pregnancy is actually 40 weeks!) to get fully ready for a deep breath in and a deep breath out.

When most of us hear those words “You’re pregnant” (particularly after years of trying), we start to think about the end. And while it’s almost like planning for mile 26 before the race gun has even gone off, your due date is a big deal. But no matter how dynamite it is to know when D-Day is, your due date is NOT dynamic. It’s pretty dead set, especially after doing fertility treatment. IVF, IUI, and all the like leave little to the imagination. There is no questioning when your insemination or transfer was performed. Be mindful of these dates and the difference in how your due date is calculated after doing fertility treatment. While we love the web (#trulyMD), we want you to be careful when searching it for your due date. Make sure to use an IVF or fertility treatment calculator. This will save you a lot of calendar crossouts and changing-of-the-date chaos.