# Pregnancy Due Date Calculator

- **URL:** https://codeasystem.com/calculators/health/pregnancy-due-date-calculator/
- **Category:** health
- **Description:** Estimate your due date from the first day of your last menstrual period using Naegele’s rule, plus your current gestational week and trimester.
- **Primary output:** Estimated due date (Naegele’s rule): Monday, March 22, 2027

## Inputs
- First day of last period (name: `lmp`, type: date, example: 2026-06-15)

## Outputs
- Estimated due date (Naegele’s rule): Monday, March 22, 2027
- Current gestational week: Week 10 of 42
- Trimester: First trimester
- Days since LMP: 69

## Formula / methodology
```
Due date = LMP + 280 days
Gestational week = ⌊days since LMP ÷ 7⌋ + 1 (clamped to 42)
```

Naegele’s rule adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. Gestational age counts from the LMP itself, so week 1 begins immediately; standard obstetric convention worldwide.

All date arithmetic uses UTC calendar days, avoiding timezone drift. Only 280 days are added regardless of cycle length; providers adjust for irregular cycles or late ovulation.

## Assumptions & limitations
- 28-day menstrual cycle with mid-cycle ovulation.
- Full term spans 37–42 weeks; only ~4% of babies arrive on the exact date.
- Ultrasound dating in the first trimester supersedes LMP estimates.

## How to use
1. **Recall your LMP**; First day of your most recent period. If cycles are irregular or you conceived while breastfeeding, the estimate is shakier.
2. **Enter the date**; The date picker returns the estimated due date instantly along with your current week and trimester.
3. **Confirm with your provider**; Bring the estimate to your first appointment; early ultrasound will refine or replace it.

## Example
With an LMP of 15 June 2026, adding 280 days gives an estimated due date in late March 2027, with gestational age advancing each Monday-free calendar day thereafter.

Result for these inputs:

```
Estimated due date (Naegele’s rule): Monday, March 22, 2027
```

## About this calculator
### Why 280 days?

Obstetric convention ages pregnancy from the LMP rather than conception, adding the roughly two pre-ovulation weeks to nature’s ~266 days from fertilization. The result is the familiar 40-week framework used in every prenatal chart.

### Due dates are midpoints, not deadlines

Spontaneous births cluster between 37 and 42 weeks, with first-time mothers skewing later. An "overdue" baby at 40+5 is statistically unremarkable; induction conversations typically wait until 41+ in uncomplicated pregnancies.

### Trimesters at a glance

- First trimester: weeks 1–13; organogenesis, highest miscarriage risk.
- Second trimester: weeks 14–27; anatomy scan, often the comfort sweet spot.
- Third trimester: weeks 28–42; growth, monitoring and preparation.

## FAQs
### My cycles are not 28 days; does this still work?

Partly. Each extra day of typical cycle length shifts ovulation (and effectively the true due date) about a day later. Mention cycle length to your provider for adjustment.

### What if I know my conception date instead?

Add 266 days to conception instead. IVF transfer dates allow precise calculation; your clinic provides the adjusted due date.

### Why does the calculator reject future dates?

Gestational age counts from an event that has happened. A future LMP means a typo or, rarely, a very early pregnancy where waiting a few days gives a valid date.

> **Disclaimer:** This calculator is for general educational purposes only and does not constitute professional advice.

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Last updated: 2026-08-23 · Version: 1.0.0 · [HTML version](https://codeasystem.com/calculators/health/pregnancy-due-date-calculator/)