Your pregnancy due date is 280 days (40 weeks) from the first day of your last menstrual period. That single date drives almost every milestone in prenatal care, from scheduling screenings to planning leave from work. You can estimate it at home in under a minute, but an early ultrasound is the most reliable way to confirm it.
The Quick Math: Naegele’s Rule
The standard formula used by doctors and online calculators alike is called Naegele’s Rule. Johns Hopkins Medicine lays it out in three steps:
- Start with the first day of your last menstrual period (LMP).
- Count back three calendar months.
- Add one year and seven days.
So if your last period started on March 10, you’d count back to December 10, then add a year and seven days to land on December 17 of the following year. That’s your estimated due date.
This formula assumes a 28-day cycle with ovulation happening right around day 14. If that describes your cycle, the math works well. If it doesn’t, you’ll need to adjust.
Why Your Cycle Length Matters
Menstrual cycles can range anywhere from 21 to 35 days and still be considered normal. The due date formula only works cleanly for a 28-day cycle because it assumes you ovulated two weeks after your period started. If your cycle is longer, say 35 days, you likely ovulated later, which pushes your true due date later than the formula suggests. A shorter cycle means earlier ovulation and a slightly earlier due date.
The simplest way to adjust: figure out how many days your cycle differs from 28, then shift the due date by that number. If your cycle is typically 32 days, add four days to whatever Naegele’s Rule gives you. If it’s 25 days, subtract three.
What If Your Periods Are Irregular
If you don’t have a predictable cycle, or you can’t remember the exact date of your last period, the LMP calculation becomes unreliable. This is common with conditions like polycystic ovary syndrome, recent use of hormonal birth control, or breastfeeding. In these situations, an early ultrasound becomes the primary tool for dating your pregnancy rather than a backup one.
If you were tracking ovulation with test strips or basal body temperature, that information can help too. Knowing your ovulation date lets you estimate conception, then add 266 days (38 weeks) to reach the due date. This bypasses the LMP entirely and can be more accurate for people with unpredictable cycles.
How Ultrasound Dating Works
An ultrasound in the first trimester is the most accurate way to pin down a due date. Between about 6 and 9 weeks, the embryo grows at a remarkably consistent rate across pregnancies, so measuring its length gives a reliable estimate of gestational age.
The American College of Obstetricians and Gynecologists (ACOG) has clear guidelines for when an ultrasound date should replace the one based on your last period. The earlier the ultrasound, the smaller the margin of error. A scan in the first trimester can be accurate to within a few days, while one in the second trimester has a wider range. If there’s a meaningful gap between your LMP-based date and the ultrasound date, your provider will typically go with the ultrasound.
Once a due date is established and confirmed by early ultrasound, it generally shouldn’t be changed later in pregnancy. Later scans reflect individual variation in fetal size more than they reflect actual gestational age.
Gestational Age vs. Fetal Age
One detail that confuses a lot of people: gestational age counts from the first day of your last period, not from conception. That means for roughly the first two weeks of your “pregnancy,” you weren’t actually pregnant yet. Conception typically happens around week 2 of gestational age.
Your baby’s actual age (sometimes called fetal age or ovulatory age) is about two weeks less than the gestational age your provider uses. When someone says they’re “8 weeks pregnant,” the embryo has really been developing for about 6 weeks. The 40-week timeline is a convention, not a biological clock that starts at fertilization.
Due Dates After IVF or Frozen Embryo Transfer
If you conceived through IVF, the calculation is more precise because the exact dates of egg retrieval and embryo transfer are known. The transfer date replaces the guesswork of figuring out when ovulation happened. For a frozen embryo transfer at the blastocyst stage (day 5, 6, or 7 embryos), the due date calculation is the same regardless of which specific day the embryo was frozen. All blastocyst-stage transfers use a single formula based on the transfer date.
Fundal Height: A Low-Tech Check Later On
Starting around 24 weeks, your provider will likely measure the distance from your pubic bone to the top of your uterus at each visit. This measurement, in centimeters, roughly matches the number of weeks you are, give or take about 3 centimeters. At 30 weeks, for example, you’d expect a measurement of around 27 to 33 centimeters.
Fundal height isn’t used to set a due date. It’s a quick screening tool to check that growth is tracking as expected. A measurement that’s significantly off might prompt an ultrasound to take a closer look, but it won’t change the due date established earlier in pregnancy.
How Precise a Due Date Really Is
A due date is an estimate, not an appointment. Only about 4 to 5 percent of babies arrive on their exact due date. Full-term birth is considered anything between 39 weeks and 40 weeks, 6 days, with many healthy pregnancies delivering a week or two on either side of the estimate. The due date is best understood as the middle of a window rather than a target.
That said, the date still matters. It determines the timing of genetic screening, glucose testing, and decisions about induction if the pregnancy goes past 41 weeks. Getting it as accurate as possible early on keeps the rest of your care on track.