Knowing one’s due date isn’t always easy. Irregular periods and fertility therapy can sometimes make figuring this out even more challenging. At University of Mississippi Medical Center, our fertility specialists are constantly trying to figure out how to provide clearer and better care, which is why we created this due date calculator. Simply enter the day and the month in the category that describes the best information you have on how you got pregnant.
For spontaneous pregnancy without monitoring or intervention, although the first day of the last period is the least accurate estimate of when you ovulated, it is close enough for many people. The day of the basal body temperature rise usually occurs the day after ovulation and a positive LH predictor kit usually occurs the day before. (Some medical conditions make LH predictor kits inaccurate, so you have to know if they work for you.) For people with short or long follicular phases (which lasts from the first day of one’s period until the day of ovulation), using just the last menstrual period isn’t very accurate, so one has to include the length of the follicular phase. Though anyone can have issues with follicular phase length, this is most important in patients with short or long menstrual cycles.
For low-tech approaches to fertility, it becomes much easier to predict when ovulation occurred. Insemination is typically performed on the day of ovulation. When giving human chorionic gonadotropin (“hCG”) to create the equivalent of an LH surge, ovulation usually occurs approximately 36 hours later. Of note, though we prefer to do inseminations around the time of ovulation, sperm are known to survive in the reproductive tract for days, so if given a few hours earlier, this is unlikely to shift fertility meaningfully.
For high-tech approaches (in vitro fertilization- IVF), when conception occurred is pretty clear. Of note, how many days after egg retrieval that embryos are returned to the body matters, which is why we adjust due date estimates based on day 3 or day 5 transfer. Much less frequently, a day 1, 2, 4 or 6 transfer will occur, in which case one should simply enter the day of the transfer and how many days after the retrieval the transfer was performed.
Finally, for the “how far along…” questions, “How far along am I today?” simply requires that you enter your due date to determine how much your pregnancy has progressed. Knowing how far along you are is critical for multiple questions, including many tests for Down syndrome, when to give steroids to reduce risks from preterm birth, whether you might be a candidate for medications to stop contractions if in preterm labor, and when to induce delivery. For asking “When will I be this many weeks?” you again enter your due date and the number of appropriate weeks. Of note, with this approach, every single week will land on the same day of the month. (If your due date is a Wednesday, you will be 28-0/7 weeks, 34-0/7 weeks, etc. on Wednesdays as well.)
For some additional trivia on due dates, only approximately 5% of women will deliver on the assigned date. Although the due date is often calculated as 40 weeks from the first day of the last period, the trend in the US is for women to deliver slightly earlier, where the average gestation is approximately 39 weeks. That being said, even though many physicians won’t stop labor if a pregnancy is past 32 to 34 weeks (or for specific medical indications), there can be benefits from postponing elective delivery until 39 weeks or later.
Good luck, and we hope your pregnancy goes wonderfully!