The due date for pregnancy is typically calculated by counting 40 weeks (280 days) from the first day of the last menstrual period (LMP). This is the most common and standard method assuming a regular 28-day cycle. Alternatively, if the exact conception date is known, the due date can be calculated by adding 266 days (about 38 weeks) to that date. For those undergoing IVF, the embryo transfer date is used with different adjustments depending on the embryo stage at transfer. Ultrasound measurements, especially in the first trimester, can also provide an accurate due date estimate.
Common Methods to Calculate Due Date
- Last Menstrual Period (LMP) Method: Add 280 days (9 months and 7 days) to the first day of your last menstrual period assuming a 28-day cycle.
- Conception Date Method: Add 266 days (38 weeks) to the date of conception if known.
- IVF Transfer Date: Add 263 days for day-three embryo transfer or 261 days for day-five transfer to the transfer date.
- Ultrasound Scan: Early ultrasound during the first trimester measures fetal size to estimate the due date, often more accurate than LMP for irregular cycles.
Adjustments for Cycle Length
- If menstrual cycles deviate from 28 days, adjust the due date by adding or subtracting the difference from 28 days.
- For irregular or very long/short cycles, ultrasound dating is generally more reliable.
Summary Formula (Naegle’s Rule)
Due Date=First day of LMP+7 days−3 months+1 year\text{Due Date}=\text{First day of LMP}+7\text{ days}-3\text{ months}+1\text{ year}Due Date=First day of LMP+7 days−3 months+1 year
Or simply add 280 days from the first day of the last period as a straightforward method. This calculation gives an estimated date; actual delivery may occur between 37 and 42 weeks of pregnancy.
