What is a reverse due date calculator?
A reverse due date calculator starts with your estimated due date and works backward to estimate key pregnancy dates. Most people use it to estimate the likely conception date and last menstrual period (LMP) when they only know the due date.
This can be helpful for planning, recordkeeping, and understanding your pregnancy timeline. It is also commonly used by people searching for a conception date calculator, pregnancy week calculator, or LMP calculator.
How this calculator works
Standard pregnancy dating
In standard obstetric dating, a full-term pregnancy is counted as 280 days (40 weeks) from the first day of your last menstrual period. Ovulation and conception usually happen about 14 days after LMP in a 28-day cycle.
- Estimated LMP = Due date minus 280 days
- Estimated conception date = Estimated LMP plus (cycle length - 14)
- Fertile window = Roughly 5 days before conception through 1 day after
Why cycle length matters
If your cycle is longer than 28 days, ovulation usually occurs later, so conception is estimated a bit later. If your cycle is shorter, conception is estimated earlier. This adjustment improves the estimate, but ultrasound measurements and clinical guidance are still more precise.
What your results include
After you click calculate, you’ll get a practical timeline with:
- Estimated first day of your last period (LMP)
- Estimated conception date
- Estimated fertile window
- Trimester milestone dates
- A status message based on today’s date (before pregnancy timeline, currently pregnant, or post due date)
Important accuracy notes
Every pregnancy is unique. A due date is always an estimate, not a guarantee. Ovulation can shift for many reasons, including stress, illness, breastfeeding, and irregular cycles.
- Early ultrasound is often the most accurate dating method.
- IVF pregnancies are usually dated from embryo transfer details.
- Cycle-based estimates can be off by several days (or more).
When to contact your clinician
This tool is for educational use and planning only. Reach out to your OB-GYN or midwife for individualized medical advice, especially if:
- Your periods are very irregular
- You are unsure about your due date
- You conceived through fertility treatment
- You have bleeding, pain, or any urgent symptoms
Quick FAQ
Can this calculator tell me the exact day I got pregnant?
No. It provides an estimate based on standard timing assumptions and your cycle length.
Is due date the day labor must start?
No. Spontaneous labor can start before or after the estimated due date. A delivery window around that date is normal.
What if I don’t know my cycle length?
Leave it at 28 days. That is the standard clinical assumption used in many basic due date and conception calculations.