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Due Date Calculator: How to Work Out Exactly How Far Along You Are
Two lines on a test, one racing heart — and immediately, two questions: when is this baby coming, and how far along am I right now? This guide explains exactly how a due date calculator works, the four dating methods doctors actually use, and what your due date really means (spoiler: it's more of a due zone).
The golden rule: 280 days from your last period
Doctors date pregnancy from the first day of your last menstrual period (LMP) — not from conception. Why? Because you know your period date for sure, while the exact day of conception is nearly impossible to pin down. So:
- Estimated due date = first day of your last period + 280 days (40 weeks)
- How far along you are = days since your last period started ÷ 7
Example: if your last period started on January 1st, your due date lands around October 8th. And 84 days after January 1st, you're exactly 12 weeks pregnant.
The 4 dating methods (and when each applies)
1) Last menstrual period — the default
Works well if your cycles are regular. If your cycle isn't 28 days, the estimate shifts: a 32-day cycle pushes the due date about 4 days later. A good pregnancy calculator adjusts for your cycle length automatically.
2) Conception date
If you know your ovulation or conception day (temperature tracking, ovulation tests), add 266 days — the count starts at fertilization, skipping the two "phantom weeks."
3) Ultrasound — the most accurate
A first-trimester scan (weeks 8–13) measures your baby's crown-rump length and dates the pregnancy to within 5–7 days. If the scan differs from your LMP date by more than a week, doctors adopt the scan date. Later scans get progressively less accurate for dating — babies start growing at different healthy rates.
4) IVF — the most precise of all
With IVF the embryo transfer date is known exactly: add 261 days for a day-5 transfer or 263 days for a day-3 transfer. No guessing involved.
Janeen calculates your due date with all four methods, then walks with you week by week — baby's size, length and weight, kick counter, symptom diary and smart checkup reminders. Free, and your data never leaves your phone.
Download Janeen freeWhat your due date actually means
Here's the part most calculators don't tell you: only about 4–5% of babies arrive on their exact due date. The realistic picture:
| When | What it's called |
|---|---|
| Before 37 weeks | Preterm — needs closer care |
| 37–38 weeks | Early term |
| 39–40 weeks | Full term — the sweet spot |
| 41 weeks | Late term |
| 42+ weeks | Post-term — usually induced |
So treat your due date as the center of a four-week window rather than an appointment. Pack the hospital bag by week 36 and let baby pick the day.
Due date FAQ
Can my due date change?
Yes — once, usually. If your first-trimester ultrasound disagrees with your period-based date by more than 7 days, your provider updates it. After that it stays fixed, even if later scans show baby measuring big or small.
How many weeks is 6 months pregnant?
Roughly weeks 23–27. Months and weeks don't map neatly (a month is ~4.35 weeks), which is why healthcare runs on weeks — and why it's worth learning to think in weeks too.
What's the most accurate due date calculator?
Any calculator using the standard formulas gives the same LMP estimate — accuracy comes from the method, not the tool. Ultrasound dating beats LMP; IVF dating beats both. The best tool is one that supports all methods and adjusts for your cycle length.
The bottom line
280 days from your last period, ultrasound wins any disagreement, and the "date" is really a zone. Calculate it once, then spend your energy on the fun part — watching your baby grow from poppy seed to watermelon, week by week. That's exactly what Janeen was built for.