Determining an estimated due date (EDD) is an essential clinical milestone in prenatal care, guiding maternal serum screenings, ultrasound anatomy scans, gestational diabetes evaluations, and delivery planning. The standard clinical convention calculates gestational age beginning from the first day of the last menstrual period (LMP) rather than the precise moment of conception.

### Tool Implementation and Calculation Mechanics The tool implements Naegele's rule with cycle-length adjustments and dual calculation modes:

- **Calculation Modes:**\n - **Last Menstrual Period (LMP) Mode:**\n Adds 280 days (40 weeks) from the start of the last menstrual period, adjusted for cycles differing from the standard 28-day baseline:\n $$\text{Due Date} = \text{LMP Date} + 280\text{ days} + (\text{Cycle Days} - 28)$$\n - **Conception Date Mode:**\n Adds 266 days (38 weeks, or 280 minus 14) directly from the known date of conception or ovulation:\n $$\text{Due Date} = \text{Conception Date} + 266\text{ days}$$ - **Gestational Age Tracking:**\n $$\text{Total Days Pregnant} = \max\left(0, \left\lfloor\frac{\text{Current Date} - \text{LMP Date}}{86,400,000\text{ ms}}\right\rfloor\right)$$\n - Gestational Weeks = $\lfloor\text{Total Days} / 7\rfloor$\n - Gestational Days = $\text{Total Days} \pmod 7$ - **Trimester Developmental Milestones:**\n - **First Trimester:** Weeks 1 through 13 ($\text{LMP} + 91\text{ days}$)\n - **Second Trimester:** Weeks 14 through 27 ($\text{LMP} + 189\text{ days}$)\n - **Third Trimester:** Weeks 28 through 40 ($\text{LMP} + 280\text{ days}$)

### Worked Numeric Example Using the tool default settings: - **Calculation Mode:** Last Menstrual Period (LMP) - **LMP Date:** 56 days (8 weeks) prior to today - **Cycle Length:** 28 days ($\text{Adjustment} = 28 - 28 = 0\text{ days}$) - **Calculation Steps:**\n 1. Estimated Due Date: $\text{LMP Date} + 280\text{ days}$ (approximately 32 weeks from today).\n 2. Current Gestational Age: $\lfloor 56 / 7 \rfloor = 8\text{ weeks, } 0\text{ days}$.\n 3. Current Trimester: **First Trimester** (ends at 13 weeks, 35 days from today).\n 4. Second Trimester Span: Begins at week 14 (day 91 from LMP) through week 27 (day 189 from LMP).

### Medical Limitations & Strict No-Diagnosis Disclaimer Naegele's rule provides a statistical estimation. Fewer than 5% of infants are delivered on their exact estimated due date; delivery between 37 and 42 completed weeks is considered normal full-term gestation. First-trimester crown-rump length (CRL) obstetric ultrasound remains the definitive clinical standard for accurate gestational age confirmation. This tool provides numerical estimates only and does not replace professional prenatal care from an OB/GYN or certified nurse-midwife. All dates process 100% locally in browser memory with zero server storage.