A clear breakdown of Naegele's rule, what your due date really means, and what's happening with your baby at every stage of pregnancy — from the first heartbeat to full term.
You typed a date into a calculator and got back a number. But where does that number actually come from — and how accurate is it?
Here's the full picture.
The standard formula used by doctors, midwives, and every due date calculator on the internet is called Naegele's Rule, named after German obstetrician Franz Karl Naegele who described it in the early 1800s.
The formula is simple:
Due date = First day of your last menstrual period (LMP) + 280 days
280 days is exactly 40 weeks. That's the assumed length of a typical pregnancy.
Why start from your last period instead of conception? Because most people don't know their exact conception date — but almost everyone knows when their last period started. By convention, pregnancy is counted from the LMP, which means you're technically "2 weeks pregnant" before conception even happens.
If you know when you conceived (for example, through IVF or tracked ovulation), you can work backwards: subtract 14 days from your conception date to get your estimated LMP, then add 280 days.
That's exactly what the "Conception Date" option in our calculator does.
Here's the honest answer: only about 5% of babies are born on their exact due date.
Studies show that most spontaneous labors happen between weeks 39 and 41. Your due date is the midpoint of a normal range, not a deadline.
Several factors affect timing:
This is the most rapid period of development. In just 12 weeks, a fertilized egg becomes a fully formed fetus with a beating heart, limbs, fingers, toes, and the beginnings of every organ system.
Week 4 — The embryo implants in the uterine wall. hCG levels are high enough to show a positive pregnancy test.
Week 6 — A heartbeat is detectable via transvaginal ultrasound. The embryo is about 6mm long — the size of a lentil.
Week 8 — Now called a fetus. About the size of a kidney bean (1.6cm). Fingers and toes are webbed but forming. The brain is developing rapidly.
Week 10 — Fingernails and toenails begin forming. The fetus can make small movements, though you won't feel them yet.
Week 12 — End of the first trimester. Risk of miscarriage drops significantly. The fetus is about 6cm long and weighs around 14g. Most major organs are formed.
Often called the "honeymoon trimester." Morning sickness typically eases, energy returns, and the baby bump becomes visible. This is when most people start to feel fetal movement.
Week 16 — The fetus can make facial expressions — squinting, frowning, grimacing. The skeleton is hardening from cartilage to bone.
Week 18–20 — The anatomy scan (also called the 20-week scan) checks all major organs and can often reveal the sex of the baby. You'll likely feel the first kicks around this time.
Week 24 — The viability milestone. Babies born at 24 weeks have a chance of survival with intensive medical support. Lung development is still the major hurdle.
Week 27 — End of the second trimester. The baby's eyes can open. Brain activity is measurable.
The final stretch. The baby gains most of its weight in this period, and the lungs mature in preparation for breathing air.
Week 28 — The baby can hear sounds from outside the womb and responds to light. Rapid eye movement (REM) sleep begins.
Week 32 — The baby practices breathing by inhaling and exhaling amniotic fluid, strengthening the lungs. Most babies have turned head-down by now.
Week 36 — Considered "early term." Most major development is complete. The baby is gaining about 30g of fat per day.
Week 37–38 — "Full term." The lungs are mature enough to breathe independently.
Week 40 — Your due date. But remember — going past 40 weeks is completely normal.
If your doctor orders a dating ultrasound in the first trimester (typically between weeks 8–13), they measure the crown-rump length of the embryo. This measurement is more accurate than LMP dating alone — to within about 5 days.
If there's a significant discrepancy between your LMP-based due date and the ultrasound measurement, your doctor will usually update your due date based on the ultrasound. This is normal and doesn't mean anything is wrong.
Your due date is a well-informed estimate, not a precise prediction. Use it as the center of a window — roughly weeks 37 to 42 — during which your baby is likely to arrive.
The most important thing is to stay in contact with your healthcare provider, attend your scheduled appointments, and trust the process.
Not sure when your little one is arriving? Use our free due date calculator — enter your last period or conception date and get your estimated due date, current week, trimester, and a full milestone timeline.
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