Your due date is 280 days (40 weeks) from the first day of your last menstrual period. That’s the standard calculation, and you can do it at home in about ten seconds. But this number is an estimate, not a guarantee. The actual date your baby arrives depends on when you ovulated, how accurate your dates are, and what an early ultrasound reveals. Here’s how each method works and which one your provider will ultimately rely on.
The Quick Math You Can Do Right Now
The most widely used formula is called Naegele’s Rule, and Johns Hopkins Medicine lays it out in three steps:
- Step 1: Find the first day of your last menstrual period (LMP).
- Step 2: Count back three calendar months from that date.
- Step 3: Add one year and seven days.
So if your last period started on March 10, you’d count back three months to December 10, then add a year and seven days to land on December 17. That’s your estimated due date. Every online due date calculator uses this same formula behind the scenes.
The catch: Naegele’s Rule assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, the estimate shifts. A woman with a 35-day cycle, for instance, likely ovulated a week later than the formula assumes, which would push her true due date about a week later as well. If you track your cycles and know they run long or short, mention the exact length to your provider so the estimate can be adjusted.
Why Ultrasound Dating Is More Reliable
Many people aren’t sure of their last period, have irregular cycles, or were on hormonal birth control recently. That’s why an early ultrasound is the gold standard for pinning down a due date. During a first-trimester scan, the sonographer measures your baby from the top of the head to the bottom of the spine. This measurement, called the crown-rump length, correlates closely with gestational age because embryos grow at a remarkably predictable rate in the early weeks.
The accuracy depends on timing. Before 14 weeks, an ultrasound date is accurate to within 5 to 7 days. Between 14 and 22 weeks, accuracy drops to within 7 to 10 days. After that window, the margin of error widens further because babies start growing at more individual rates. This is why your provider will typically schedule a dating scan in the first trimester if there’s any uncertainty.
If the ultrasound date and your LMP-based date are close (within about a week for a first-trimester scan), your provider will generally stick with the LMP date. If the two dates disagree by more than that margin, the ultrasound measurement takes priority. Once a due date is set this way, it usually isn’t changed later in pregnancy, even if a later scan suggests a slightly different size.
If You Conceived Through IVF
IVF dating is actually more precise than natural conception dating because the exact day of fertilization and embryo transfer is known. Instead of counting from a last menstrual period with a built-in two-week buffer for ovulation, the calculation starts from the date of embryo transfer. For a Day 5 embryo transfer (the most common type with frozen embryos), your provider counts 263 days forward, or equivalently treats the transfer date as if you’re already 2 weeks and 5 days pregnant and counts to 40 weeks from there. Whether the embryo was frozen on Day 5, Day 6, or Day 7, the calculation is the same once it’s transferred at the blastocyst stage.
First Pregnancy vs. Later Pregnancies
Your due date calculation doesn’t change based on whether this is your first baby or your fourth, but the statistical likelihood of when you’ll deliver does. Research on pregnancy duration found that first-time mothers carry an average of about 5 days longer than women who have given birth before. The median pregnancy length for a first baby was 274 days from ovulation, compared to 269 days for subsequent babies. That’s a real and statistically significant difference, though it’s not large enough to change your official due date. It just means first-time parents should mentally prepare for the possibility of going past their date.
How Precise Your Due Date Really Is
A due date is the midpoint of a range, not a target. Full-term pregnancy spans from 39 weeks through 40 weeks and 6 days, and babies born anywhere from 37 to 42 weeks are broadly considered within the normal window. Because induction has become common at or near 40 weeks in the U.S. and other Western countries, it’s difficult to say exactly what percentage of pregnancies would naturally hit their due date. What’s clear is that the majority of babies arrive in the two weeks before or after the estimated date, not on the date itself.
The best thing you can do for an accurate due date is get an early ultrasound, give your provider an honest accounting of your cycle length and certainty about your last period, and treat the date as a useful planning tool rather than a firm appointment. Your provider will use the most reliable piece of information available, whether that’s a confident LMP, a first-trimester scan, or an IVF transfer date, and set a single estimated due date early in your care.