A simple guide to estimating the due date and gestational age, as well as ovulation day and the fertile window, starting from the first day of the last menstrual period.
Whether you are planning a pregnancy or following an existing one, understanding how key dates are estimated helps you plan with confidence. Every calculation starts from one essential piece of information: the first day of the last menstrual period (LMP).
Calculated using the classic Naegele rule:
> Estimated due date = first day of the last period + 280 days (40 weeks).
This rule assumes a regular 28-day cycle with ovulation on day 14.
Measured as the number of weeks and days since the first day of the last period, usually shown as something like "12 weeks and 3 days", and divided into three trimesters:
| Trimester | Weeks |
|---|---|
| First | 0 – 13 |
| Second | 14 – 27 |
| Third | 28 – 40+ |
Depends on your cycle length, estimated by subtracting about 14 days (the roughly fixed luteal phase) from the next expected period:
> Ovulation ≈ first day of the last period + (cycle length − 14).
The most fertile days span from 5 days before ovulation through about one day after, because sperm can survive up to five days while the ovum lives about 12 to 24 hours.
Try it now: Use the [Pregnancy & Ovulation Calculator](https://www.medclac.com/#open=pregnancy_ovulation) to estimate your due date and gestational age, or your ovulation day and fertile window.
Disclaimer: This content is for educational guidance only and does not replace professional medical consultation.
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