Pregnancy is calculated from the first day of your last menstrual period (LMP), not from the day you actually conceived. This means you’re technically considered “pregnant” for about two weeks before fertilization even happens. The standard calculation puts a full-term pregnancy at 40 weeks, or 280 days, from that starting point.
Why the Clock Starts Before Conception
This is the part that confuses most people. Ovulation and fertilization typically happen about two weeks after the start of a menstrual period, so the pregnancy calendar already has roughly 14 days built in before a sperm ever meets an egg. Doctors use this system because most people can recall when their last period started, while the exact day of conception is rarely known with certainty.
This creates two different “ages” for a pregnancy. Gestational age counts from the last menstrual period and is the number you’ll hear at every appointment. Fetal age (sometimes called fertilization age) counts from actual conception and runs about two weeks behind. When your provider says you’re 10 weeks pregnant, the embryo has really been developing for closer to 8 weeks.
The Standard Formula
The most widely used calculation is called Naegele’s rule. Johns Hopkins Medicine outlines it in three steps:
- Take the first day of your last menstrual period.
- Count back three calendar months.
- Add one year and seven days.
So if your last period started on March 1, you’d count back to December 1, then add a year and seven days to land on December 8 as the estimated due date. The formula assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are shorter or longer than 28 days, the estimate shifts accordingly, because ovulation would have happened earlier or later than the formula expects.
How Irregular Cycles Change the Math
If your cycle is consistently 35 days instead of 28, you likely ovulate around day 21 rather than day 14. That extra week means Naegele’s rule would place your due date about a week too early. Clinicians adjust by adding the difference between your typical cycle length and 28 days to the standard estimate. A 35-day cycle adds 7 days; a 24-day cycle subtracts 4.
For people whose cycles are truly unpredictable, varying by more than a week from month to month, the LMP method becomes unreliable. In those cases, an early ultrasound is the primary tool for establishing how far along the pregnancy is.
Ultrasound Dating
A first-trimester ultrasound, done up to 13 weeks, is the most accurate method to confirm or revise a due date. The technician measures the embryo from head to rump (called crown-rump length), and between 9 and 13 weeks, the embryo grows so rapidly and uniformly that this measurement can pin down gestational age within just a few days.
Later in pregnancy, dating scans rely on head circumference and other measurements, but natural variation between babies increases as they grow. A second-trimester scan is less precise, and by the third trimester, size differences between healthy fetuses make ultrasound dating far less useful. The American College of Obstetricians and Gynecologists considers any pregnancy without a dating ultrasound before 22 weeks to be “suboptimally dated.”
When LMP and Ultrasound Disagree
It’s common for the date calculated from your last period and the date suggested by ultrasound to be a few days apart. When the gap is small, providers typically stick with the LMP-based date. When the discrepancy is larger, the ultrasound date takes priority, especially if the scan was done in the first trimester. Your provider will assign a single “official” due date, and that estimate is what appears on the birth certificate as the basis for gestational age at delivery.
How IVF Due Dates Are Calculated
For pregnancies conceived through assisted reproduction, the calculation is more precise because the exact timing is known. The formula works backward from the embryo transfer date. If a day-5 embryo (blastocyst) was transferred, you subtract 5 days to find the equivalent conception date, then add 266 days to reach the due date. For a day-3 embryo, you subtract 3 days instead. ACOG guidelines state that when a pregnancy results from assisted reproductive technology, this method should be used over any other to assign the due date.
The Trimester Breakdown
Once the due date is set, the 40-week timeline divides into three trimesters:
- First trimester: conception through 12 weeks
- Second trimester: 13 through 27 weeks
- Third trimester: 28 through 40 weeks
These aren’t perfectly equal in length, and you’ll notice the first trimester is slightly shorter. Each stage corresponds to major developmental milestones. Organs form in the first trimester, the baby grows rapidly in the second, and the lungs and brain mature in the third.
How Accurate the Due Date Really Is
Only about 5% of babies are born on their estimated due date. The calculation gives you a target, but healthy full-term births can happen anywhere from 37 to 42 weeks. Think of the due date as the center of a window rather than a deadline. Most first-time parents go slightly past it.
The 40-week estimate is a population average, not a biological rule. Individual pregnancies vary based on genetics, the length of your natural cycle, whether it’s your first baby, and factors researchers still don’t fully understand. Your due date is the single best guess available, but it was always designed to be an estimate.