The simplest way to figure out how far along you are in pregnancy is to count from the first day of your last menstrual period (LMP). Pregnancy is dated from that day, not from conception, which means you’re technically “two weeks pregnant” before you even ovulate. From there, a full-term pregnancy lasts about 40 weeks. If you don’t remember your last period or have irregular cycles, an early ultrasound is the most accurate alternative.
Counting From Your Last Period
Start with the first day of your most recent period. Count forward from that date in weeks and days to today, and that’s how far along you are. A pregnancy app or online calculator can do this instantly, but the math is straightforward: if your last period started eight weeks ago, you’re eight weeks pregnant.
This same starting date is used to estimate your due date. The standard formula, known as Naegele’s Rule, works in three steps: take the first day of your last period, count back three calendar months, then 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 a due date of December 17. This formula assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter, your actual ovulation date shifts, and your true gestational age may be off by several days.
Why Ultrasound Is More Accurate Early On
A first-trimester ultrasound (before 14 weeks) is the most precise way to date a pregnancy. It measures the embryo from head to tailbone, and that measurement correlates tightly with gestational age, accurate to within about three days. The earlier the scan, the better, because embryos grow at nearly identical rates in the first weeks. Later in pregnancy, individual growth differences make sizing less reliable for dating purposes.
If your ultrasound date and your period-based date agree closely, your due date stays the same. But when there’s a meaningful gap between the two, the ultrasound wins. The American College of Obstetricians and Gynecologists lays out specific thresholds: if an ultrasound between 14 and 16 weeks differs from your period-based date by more than 7 days, or one between 16 and 22 weeks differs by more than 10 days, the due date should be changed to match the ultrasound. Between 22 and 28 weeks, the threshold widens to 14 days.
If You Don’t Know Your Last Period
Irregular cycles, breakthrough bleeding, or simply not tracking your period can make LMP dating unreliable. In those situations, an early ultrasound becomes the primary dating tool rather than a backup. The same measurement principles apply: the earlier the scan, the more accurate. If you’re unsure of your dates, scheduling an ultrasound as soon as possible gives you the clearest answer.
Some people conceive while on hormonal birth control or while breastfeeding, situations where there may not be a “last period” to count from at all. If that applies to you, ultrasound dating is essentially the only reliable method.
Dating an IVF Pregnancy
If you conceived through IVF, the math is more precise because you know the exact date of embryo transfer. The formula subtracts the embryo’s age at transfer to find the equivalent conception date, then adds 266 days for the due date. A day-3 embryo transfer means conception is counted as three days before transfer day. A day-5 blastocyst transfer puts conception five days before. From there, gestational age is calculated the same way as any other pregnancy, just with a more exact starting point.
Physical Signs That Roughly Track Your Weeks
Your body offers some general clues about how far along you are, though none are precise enough to replace a calendar or ultrasound. The top of the uterus (the fundus) rises in a predictable pattern as pregnancy progresses. At around 12 weeks, it sits at the level of your pubic bone. By 20 weeks, it reaches your belly button. At about 36 weeks, it climbs as high as your breastbone before the baby drops lower in preparation for birth.
After 20 weeks, a tape measure across the belly from the pubic bone to the top of the uterus gives a “fundal height” in centimeters that roughly matches the number of weeks pregnant. A measurement of 28 centimeters, for example, suggests about 28 weeks. This is a screening tool, not a dating method, but it can flag a significant mismatch worth investigating.
What About hCG Levels and Home Tests?
The pregnancy hormone hCG rises rapidly in early pregnancy, peaking around weeks 9 to 10 before declining. While certain patterns can hint at general timing, the normal range at any given week is enormous. At 9 weeks, for instance, hCG levels in a healthy pregnancy can range from about 24,000 to nearly 126,000 IU/L. That overlap between weeks makes hCG unreliable for pinpointing how far along you are.
Some digital home pregnancy tests display a “weeks since conception” estimate (typically 1–2, 2–3, or 3+ weeks). These are based on hCG concentration in urine. One clinical study found 93% agreement between the test’s estimate and the time since ovulation when normal measurement variability was accounted for. That’s reasonably accurate for a rough sense of timing in the earliest weeks, but it’s not a substitute for a proper dating scan. The estimate is displayed in weeks since conception, which is about two weeks behind the standard gestational age count that starts from your last period.
How Precise Is Any Due Date?
No method pins down the exact day your baby will arrive. The 40-week due date is a statistical midpoint, not a deadline. About half of first-time mothers give birth by 40 weeks and 5 days, meaning the other half go longer. For people who’ve given birth before, that midpoint is slightly earlier, around 40 weeks and 3 days. Because labor induction is now common at or before 40 weeks in many countries, the natural spread of delivery dates is hard to study cleanly, but the pattern is clear: your due date is a best guess within a window of several weeks.
What matters most for your care is having a consistent gestational age established early. Whether it comes from your last period, an ultrasound, or your transfer date, that number anchors every screening test, growth check, and timing decision for the rest of pregnancy. The earlier it’s set, the more reliable everything built on top of it will be.