Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. That means by the time you miss a period and get a positive test, you’re typically already considered about 4 weeks pregnant. Counting from your LMP gives you a quick starting estimate, but an early ultrasound is the most reliable way to pin down exactly how far along you are.
Counting From Your Last Period
The standard method counts forward from the first day of your most recent period. A full-term pregnancy is 280 days, or 40 weeks, from that date. This might feel counterintuitive because you weren’t actually pregnant during the first two weeks of that count. Ovulation and conception typically happen around day 14 of a 28-day cycle, so gestational age runs roughly two weeks ahead of how long the embryo has actually been developing.
To estimate your due date at home, you can use a simple three-step formula known as Naegele’s Rule:
- Start with the first day of your last period.
- Count back 3 calendar months.
- Add 1 year and 7 days.
For example, if your last period started on March 10, you’d count back to December 10, then add a year and 7 days, giving you a due date of December 17. From there, you can figure out how many weeks have passed since your LMP to know where you are right now.
This formula assumes a regular 28-day cycle with ovulation on day 14. It works well for many people, but it has real limitations. If you don’t remember exactly when your last period started, if your cycles are irregular, or if you weren’t tracking closely, the estimate can be off by a week or more.
What If Your Cycles Are Irregular?
If your cycles run significantly longer or shorter than 28 days, the LMP method becomes less reliable. A person with a 35-day cycle, for instance, likely ovulated around day 21 rather than day 14, which means gestational age calculated from the LMP would overestimate how far along they are by about a week. The reverse is true for shorter cycles.
Some online due date calculators let you adjust for cycle length, which can improve accuracy. But if your periods have always been unpredictable, or you were on hormonal birth control shortly before conceiving and aren’t sure when normal cycles resumed, an ultrasound becomes especially important for establishing dates.
Ultrasound: The Most Accurate Method
An ultrasound in the first trimester is the gold standard for determining how far along you are. During this scan, a technician measures the embryo from head to rump (called crown-rump length), and that measurement correlates closely with gestational age. In the early weeks, embryos grow at a very consistent rate regardless of genetics, which is what makes this measurement so precise.
The earlier the ultrasound, the more accurate it is. A scan between about 7 and 13 weeks can estimate gestational age to within a few days. After that window, the margin of error grows because babies start growing at different rates based on individual factors. A second-trimester ultrasound is still useful but less exact, and by the third trimester, size differences between healthy babies are large enough that ultrasound dating becomes significantly less reliable.
Most prenatal care includes a dating scan around 12 weeks. If the ultrasound measurement differs meaningfully from what your LMP suggests, your provider will typically adjust your due date and gestational age to match the scan. This adjusted date is what gets used for all future care decisions.
Pregnancies From Fertility Treatment
If you conceived through IVF or another assisted reproductive technology, dating works differently. The American College of Obstetricians and Gynecologists recommends using the known age of the embryo and the date of transfer to assign gestational age, rather than relying on LMP or ultrasound estimates. This gives the most precise dating possible because the exact day of fertilization is known.
Other Ways to Estimate Gestational Age
Blood tests that measure hCG (the hormone your body produces during pregnancy) can confirm that a pregnancy is progressing, but they aren’t useful for precise dating. The normal range at any given week is enormous. At 6 weeks, for instance, hCG levels can fall anywhere from 200 to 32,000 units. At 7 weeks, the range stretches from 3,000 to 160,000. Two people at the exact same gestational age can have wildly different levels, so hCG tells you very little about how far along you are.
Later in pregnancy, your provider may measure fundal height, which is the distance from your pubic bone to the top of your uterus. After about 24 weeks, this measurement in centimeters roughly matches the number of weeks pregnant you are, give or take about 3 centimeters. A measurement of 30 centimeters, for example, suggests you’re around 30 weeks along. This is a quick check rather than a precise dating tool, and it’s mainly used to track whether the baby is growing as expected once gestational age is already established.
Why Getting the Dates Right Matters
Accurate dating isn’t just about knowing when to expect delivery. Gestational age determines when you’re offered specific prenatal screenings, some of which are only accurate within certain week ranges. It also affects how your provider interprets the baby’s growth and development at each visit. If labor starts early, knowing exactly how far along you are helps the medical team assess the baby’s readiness. And if your pregnancy goes past your due date, accurate dating helps determine when induction should be considered.
If you’re trying to figure out your gestational age right now, start by counting the weeks since the first day of your last period. That gives you a working number. When you have your first ultrasound, that estimate may shift by a few days to a week. The ultrasound-adjusted date is the one to go by from that point forward.