Pregnancy is calculated by counting 280 days (40 weeks) from the first day of your last menstrual period, or LMP. This is the standard method used by doctors worldwide, and it means your pregnancy “starts” about two weeks before you actually conceived. That sounds counterintuitive, but it exists because most people can recall when their period started more reliably than when ovulation or fertilization occurred.
The Standard Formula: Naegele’s Rule
The most widely used calculation is called Naegele’s Rule, and it works in three steps. First, identify the first day of your last menstrual period. Then count back three calendar months from that date. Finally, add one year and seven days. The result is your estimated due date.
For example, if your last period started on March 10, 2025, you’d count back three months to December 10, 2024, then add one year and seven days to get December 17, 2025. This formula assumes a 28-day menstrual cycle with ovulation happening on day 14. If that describes your cycle, it’s a reliable starting point. If it doesn’t, adjustments are needed.
Why Dating Starts Before Conception
Pregnancy dating uses what’s called gestational age, which begins on the first day of your last period. Your baby’s actual age (fetal age) is roughly two weeks younger, since conception typically happens around ovulation at mid-cycle. Clinicians use gestational age by convention because the LMP is an observable date, while the moment of conception is not. Every milestone you hear about, from the 12-week scan to the 40-week due date, uses this gestational age system.
Adjusting for Irregular Cycles
Naegele’s Rule can be off by days or even weeks if your cycle isn’t 28 days. A longer cycle means you likely ovulated later than day 14, pushing your real conception date forward. A shorter cycle means the opposite. One alternative, known as Wood’s method, builds in corrections. It uses a similar LMP-based formula but adds days for cycles longer than 28 or subtracts days for shorter ones. It also accounts for whether this is your first pregnancy or a subsequent one, since first pregnancies tend to run slightly longer.
If your cycles are very irregular, or if you’re unsure of your LMP entirely, your provider will rely more heavily on ultrasound dating instead.
How Ultrasound Refines the Estimate
An early ultrasound is the most accurate way to confirm or adjust your due date. During the first trimester, the technician measures the baby from head to rump (called crown-rump length), and because embryos grow at a very predictable rate in those early weeks, this measurement closely estimates gestational age. The earlier the scan, the more precise it is.
If ultrasound dating and LMP dating agree closely, your due date stays the same. If they differ significantly, your provider will typically go with the ultrasound. Later in pregnancy, ultrasound measurements become less reliable for dating because babies start growing at individual rates. By the third trimester, size differences between babies of the same gestational age are wide enough that a scan can’t pinpoint your dates with much precision.
Calculating Due Dates After IVF
If you conceived through IVF, the calculation is more straightforward in some ways because you know exactly when the embryo was transferred. Rather than estimating backward from a last period, the due date is calculated forward from the transfer date. For frozen embryo transfers at the blastocyst stage (day 5, 6, or 7 embryos), the calculation is the same regardless of the specific day, since they’re all considered blastocyst-stage transfers. Your fertility clinic will typically provide a precise due date at your first appointment.
Physical Checkups as a Rough Guide
Once you’re past 20 weeks, your provider has a simple hands-on method for checking whether the baby’s growth matches your dates. They measure the distance from your pubic bone to the top of your uterus (called fundal height) with a tape measure. After 20 weeks and up through about 36 weeks, the measurement in centimeters should roughly equal the number of weeks you are pregnant, plus or minus two centimeters. So at 32 weeks, a measurement between 30 and 34 centimeters is considered normal. This isn’t used to set a due date, but it helps confirm that everything is tracking as expected.
How Precise Is the Due Date, Really?
Your due date is an estimate, not an appointment. Full-term birth is considered anywhere from 39 to 40 weeks, and most babies arrive within a window around that mark rather than on the exact predicted day. Because induction is common at or before 40 weeks in Western countries, it’s difficult to know what percentage of pregnancies would naturally hit the due date on the nose. The 280-day number represents the average length of pregnancy, not a guarantee for any individual.
What matters most is having a reliable estimated date so your provider can track growth milestones, schedule appropriate screenings, and make informed decisions if your pregnancy goes significantly past that window. Whether your dates come from an LMP calculation, an early ultrasound, or an IVF transfer date, the goal is the same: a consistent reference point for the months ahead.