How due dates are calculated (Naegele's rule)
Naegele's rule — add 40 weeks (280 days) to the first day of your last menstrual period (LMP) — is the gold-standard calculation for an estimated due date (EDD). It was codified by German obstetrician Franz Karl Naegele in the early 1800s and hasn't fundamentally changed since, because the math is simple and the result is accurate enough for most clinical purposes.
The 280-day figure assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, this calculator adjusts the due date by the difference. A 32-day cycle pushes the due date 4 days later; a 24-day cycle pulls it 4 days earlier.
LMP vs. conception date
Obstetricians use the LMP as the reference because it's a date almost everyone remembers. Conception actually happens roughly 2 weeks after LMP, at ovulation, which is why "40 weeks pregnant" translates to about 38 weeks of actual embryo development. If you know your conception date (IVF transfer, known cycle day with ovulation testing), you'll get a more precise estimate by using that method.
What trimester am I in?
- First trimester: weeks 1–12. Organs develop; miscarriage risk highest; morning sickness common.
- Second trimester: weeks 13–26. Energy returns; first movements felt; anatomy scan at ~20 weeks.
- Third trimester: weeks 27–40+. Rapid growth; back pain and fatigue common; birth prep.
Key pregnancy milestones
- ~Week 6: first prenatal appointment; heartbeat typically detectable.
- ~Week 10: NIPT (noninvasive prenatal testing) blood draw available.
- ~Week 12: end of first trimester; miscarriage risk drops substantially.
- ~Week 16–22: first perceptible fetal movements ("quickening"). Earlier for second+ pregnancies.
- Week 20: anatomy scan; sex determination if desired.
- Week 24: viability milestone — around 50% survival with NICU care if delivered now.
- Week 28: third trimester begins; glucose screening for gestational diabetes.
- Week 36: GBS (Group B strep) screening.
- Week 37: pregnancy considered full-term.
- Week 40: estimated due date.
- Week 41+: post-term; induction is typically discussed.
How accurate is an EDD?
Only about 5% of babies arrive on their exact due date. Roughly 50% arrive within a week of it, and 80% within 10 days. An early ultrasound (dating scan in the first trimester) can refine the estimate by a few days and is considered more accurate than LMP if they disagree by more than 7 days.
Tracking pregnancy health
During pregnancy, the water intake calculator applies the +10 oz pregnancy adjustment for you, and the blood pressure calculator is worth returning to each prenatal visit — preeclampsia screening depends on BP trends. If you were tracking your cycle to conceive, the ovulation calculator is the pre-pregnancy companion to this tool.
Common first-trimester concerns
- Nausea: affects ~70%. Small, frequent meals; ginger; vitamin B6. Persistent severe nausea (hyperemesis) warrants medical attention.
- Fatigue: progesterone surge. Prioritize sleep — see the sleep calculator.
- Spotting: common but worth mentioning to your provider.
- Weight: typical gain is 0–5 lb in T1; 1 lb/week after.
What this calculator can't do
This tool doesn't account for IVF transfer dates (your clinic will give you a more precise EDD), multiple pregnancies (twins are typically delivered earlier), irregular cycles (early ultrasound dating is more accurate), or medical conditions that change expected delivery. Use the result as a starting point and defer to your provider's estimate if it differs.
FAQ
My provider's EDD differs by a week — whose is right?
Your provider's. If an early ultrasound was done, it takes precedence over LMP when they disagree by more than 7 days. This is standard ACOG guidance.
Can I accelerate labor after 40 weeks?
Walking and sex are often suggested; neither is strongly evidence-backed. Membrane sweeping and medical induction, offered by your provider around week 41, are the reliable options.
What if I don't know my LMP?
Use the conception method if you know that, or rely on an early ultrasound which can date the pregnancy within ±5 days.
Is the due date calculation different for IVF?
Yes. IVF clinics calculate from the embryo transfer date + a known embryo age, which is more precise than LMP-based estimates. Use your clinic's figure.