Pregnancy & Due Date Calculator

Pregnancy Dating and Methods

A standard pregnancy timeline spans 40 weeks, counting from the first day of the last menstrual period. While it represents a baseline, only about 5% of babies arrive exactly on their estimated due date. Knowing your gestational progress and due date helps you track developmental milestones and plan medical care.

How to use this tool

1. Select the **Calculation Method** (Last Period, Conception Date, or Due Date).
2. Enter your **Date** matching the chosen method.
3. If using the *Last Period* method, select your average **Menstrual Cycle Length** (defaults to 28 days).
4. Click **Calculate Due Date** to view your estimated due date, conception date, current gestational age (weeks and days), and detailed developmental milestones.

Understanding the formulas

• Naegele's Rule (LMP Method): Counts exactly 280 days (40 weeks) from the first day of your Last Menstrual Period (LMP). If your average cycle is longer or shorter than the typical 28 days, the calculator applies an adjustment offset: $$ \text{Due Date} = \text{LMP} + 280\text{ Days} + (\text{Cycle Length} - 28)\text{ Days} $$
• Conception Date Method: Conception typically occurs during ovulation, about 2 weeks after the LMP. The due date is calculated by adding 266 days (38 weeks) directly to the conception date: $$ \text{Due Date} = \text{Conception Date} + 266\text{ Days} $$
• Reverse Due Date Method: If you already have a target due date (e.g. from an ultrasound scan), the calculator counts backward by 266 days to find the conception date, and 280 days to estimate your LMP date.

Worked Examples

Tips and common mistakes

• Cycle Length Variation: If your cycle is irregular, calculations based on LMP can be less accurate. The average cycle length input is crucial for correcting the offset.
• Actual Delivery Dates: Remember that the calculated due date is only an estimate. Babies can arrive safely anywhere from week 37 to week 42.
• Ultrasound dating: Early ultrasounds (in the first trimester) are considered the most accurate way to date a pregnancy and can override LMP calculations.

Frequently asked questions