The Pregnancy Due Date Calculator estimates your expected delivery date (EDD) based on the first day of your last menstrual period (LMP) or your known conception date. It also provides a timeline of your trimesters to help you track fetal development and key prenatal milestones.
How It Works
The calculator determines your estimated due date using Naegele's Rule, the standard obstetric method for pregnancy dating. It assumes a standard 28-day menstrual cycle and a gestation period of 280 days (40 weeks) from the first day of the last menstrual period, or 266 days (38 weeks) from conception.
Formula & Calculation Logic
Calculating by Last Menstrual Period (LMP)
If you have a regular 28-day cycle, the formula (Naegele's Rule) is:
Alternatively, this is equivalent to adding exactly 280 days to the LMP:
Calculating by Conception Date
If you know the exact date of conception (or IVF transfer date), the formula adds 266 days (38 weeks):
Cycle Length Adjustments
Naegele's rule assumes a standard 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the calculator adjusts the expected ovulation date:
- Longer than 28 days: The EDD is pushed back by the difference in days. (e.g., for a 30-day cycle, add 2 days to the EDD).
- Shorter than 28 days: The EDD is moved forward by the difference in days. (e.g., for a 26-day cycle, subtract 2 days from the EDD).
Pregnancy Trimesters
A typical pregnancy is divided into three trimesters, measured from the LMP:
| Trimester | Timeline | Development Milestones | |---|---|---| | First Trimester | Weeks 1 - 13 | Early organ development, heart rate establishment, formation of neural tube. | | Second Trimester | Weeks 14 - 27 | Fetal movement ("quickening"), sex determination via ultrasound, significant growth. | | Third Trimester | Weeks 28 - Birth | Rapid weight gain, lung maturation, settling into the head-down position for birth. |
Practical Considerations & Limitations
- Estimates Only: An estimated due date is exactly that—an estimate. Only about 4-5% of babies are born on their exact due date. A normal full-term delivery can occur anywhere between 37 and 42 weeks.
- Ultrasound Accuracy: Early first-trimester dating ultrasounds (measuring the crown-rump length) are generally considered the most accurate method for determining gestational age, potentially overriding LMP calculations if the dates differ significantly (typically by more than 5-7 days).
- Irregular Cycles: If your menstrual cycles are highly irregular, calculating by LMP may be less accurate. In such cases, an ultrasound is recommended for precise dating.
Frequently Asked Questions
What if I don't know my conception date?
Most healthcare providers use the first day of your Last Menstrual Period (LMP) to calculate your due date, as this date is usually easier to remember and track than the exact date of conception.
How is the EDD calculated for IVF?
For In Vitro Fertilization (IVF), the due date is calculated based on the embryo transfer date and the age of the embryo (e.g., a 3-day or 5-day transfer).
- For a 3-day embryo transfer: EDD = Transfer Date + 266 days - 3 days
- For a 5-day embryo transfer: EDD = Transfer Date + 266 days - 5 days
Are these calculations private?
Yes. The calculator processes your dates locally in your browser. No health information is saved, stored, or transmitted to any servers.