- What is Naegele's rule?
- Naegele's rule is the formula clinicians have used since the 1800s to estimate pregnancy due date: take the first day of the last menstrual period (LMP) and add 280 days (40 weeks). It assumes a 28-day cycle and ovulation on day 14, so 14 days from LMP to conception plus 266 days of fetal development. It's a rule of thumb, not a precise prediction — only about 4% of babies arrive on the exact date.
- Why is it counted from my last period instead of conception?
- Most people know the date their last period started; very few know the exact date of conception. Counting from LMP is consistent, easy to ask about, and that's the convention every doctor, app, and chart uses. Gestational age (weeks since LMP) is roughly two weeks longer than fetal age (weeks since conception). When someone says they're 'six weeks pregnant,' the embryo is around four weeks old.
- Is this an estimate or precise?
- An estimate. Cycle length varies between people and between cycles — a 28-day cycle is an average, not a law. If you ovulate later than day 14, your real due date is later than the calculator says, and vice versa. The clinical workflow is to start with LMP, then refine the date once a first-trimester ultrasound has measured the embryo directly. Ultrasound dating before about week 13 is generally accurate within a few days.
- How are the trimesters defined?
- The first trimester is weeks 1 through 12. The second trimester is weeks 13 through 26. The third trimester is weeks 27 through 40. Some sources use slightly different cutoffs (e.g. first trimester ending at week 13), but the common convention is 1-12, 13-26, 27-40. Each trimester is associated with different developmental milestones, symptoms, and screenings.
- What's '12w 3d' mean?
- 12 weeks and 3 days of pregnancy, counted from the first day of your last period. It's the standard way to express gestational age — you'll see it on ultrasound reports, in your chart, and in pregnancy books. A pregnancy is typically described as 40 weeks total, but anywhere from 37 to 42 weeks is considered full term.
- When does the third trimester start?
- Week 27. From week 27 to week 40, the baby gains most of its weight, the brain develops rapidly, and your body prepares for delivery. The third trimester is also when most discomfort tends to peak — back pain, sleep difficulty, frequent bathroom trips — and when prenatal appointments shift from monthly to weekly.
- Why does the percent never quite hit 100% if my due date passes?
- It does hit 100% — at exactly 40 weeks past LMP. If you go past your due date, the percent caps at 100% (the calculator doesn't show 101%, because the pregnancy hasn't gotten any longer — you've just arrived at the post-term window). About 60% of first pregnancies and 40% of subsequent pregnancies last past the due date.
- Can I use this if my cycle isn't 28 days?
- The headline figures assume the standard 28-day cycle that Naegele's rule is built on. If your typical cycle is longer or shorter, your true ovulation day is later or earlier than day 14, which shifts the real due date by the same amount. The simplest correction is to use the Due Date Calculator (it accepts a cycle length input), get the adjusted due date, then come back here and switch to 'From due date' mode.
- Is my data saved or sent anywhere?
- No. The calculation runs entirely in your browser using JavaScript — nothing is sent to a server, logged, or stored. Nothing about your pregnancy leaves your device.
