Pregnancy Due‑Date & Week Calculator

Understanding Pregnancy Due Dates and Gestational Age

Pregnancy due date calculation is both an art and a science, combining medical knowledge with individual variations. Understanding how due dates are determined and what factors can influence them helps expectant parents prepare for their baby's arrival.

Methods of Due Date Calculation

Last Menstrual Period (LMP) Method:

This is the most commonly used method, based on Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day menstrual cycle with ovulation occurring on day 14. However, many women have cycles that vary in length, which can affect accuracy.

Ultrasound Dating:

Early ultrasound scans (performed between 8-12 weeks) can provide more accurate dating by measuring the baby's crown-rump length. This method is particularly useful when the LMP date is uncertain or when the woman has irregular cycles.

Conception Date Method:

If you know the exact date of conception (through fertility treatments or careful tracking), you can add 266 days (38 weeks) to that date. However, this method is rarely used in clinical practice due to the difficulty of pinpointing exact conception.

Factors That Can Affect Due Date Accuracy

Menstrual Cycle Variations:

• Irregular cycles make LMP dating less reliable
• Longer or shorter cycles than the standard 28 days
• Recent hormonal contraception use

Individual Pregnancy Factors:

• Maternal age and health conditions
• Previous pregnancy history
• Multiple pregnancies (twins, triplets)
• Baby's growth rate and development

External Factors:

• Stress and lifestyle factors
• Maternal nutrition and weight
• Environmental factors

Understanding Gestational Age vs. Fetal Age

Gestational Age: Measured from the first day of the last menstrual period (what this calculator shows)

Fetal Age: The actual age of the developing baby (typically 2 weeks less than gestational age)

This difference exists because conception typically occurs about 2 weeks after the start of the last menstrual period, but medical professionals use gestational age for consistency and because LMP is usually more reliably known than conception date.

Important Medical Disclaimers

⚠️ This calculator provides estimates only:

• Results should never replace professional medical advice
• Always consult with your healthcare provider for accurate dating
• Regular prenatal care is essential for monitoring pregnancy progress
• Any concerns about pregnancy should be discussed with a medical professional

When to Seek Medical Attention:

• If you're unsure about your LMP date
• If you have irregular menstrual cycles
• If you experience any unusual symptoms
• For confirmation of pregnancy and proper prenatal care planning

Remember that every pregnancy is unique, and your healthcare provider will monitor your specific situation throughout your pregnancy journey.