Pregnancy Estimated Due Date (EDD

Clinical Obstetrics: Calculating Estimated Due Dates & Gestational Age

Obstetric gestational age is measured from the first day of the last menstrual period (LMP) rather than from the moment of conception, making a standard full-term human pregnancy approximately 40 weeks (280 days):

• Naegele's Rule & Cycle Length Adjustments: Naegele's Rule assumes an idealized 28-day cycle with ovulation occurring on Day

• Parikh's modification adjusts for women with shorter (e.g. 24-day) or longer (e.g. 35-day) cycles by adding or subtracting the deviation from 28 days.

• In Vitro Fertilization (IVF) Dating: IVF pregnancies provide exact fertilization timing. A Day 5 embryo transfer (blastocyst) calculates EDD by adding 261 days to the transfer date, representing the most precise gestational dating possible.

• Trimester Breakdown:

• First Trimester: Weeks 1 through 13 + 6 days (organogenesis, neural tube closure).

• Second Trimester: Weeks 14 through 27 + 6 days (rapid skeletal growth, anatomical scan at 18–20 weeks).

• Third Trimester: Weeks 28 through delivery (pulmonary surfactant maturation, rapid adipose deposition).

How to Use This Calculator

Enter your Calculation Method — select from First Day of Last Menstrual Period (LMP - Naegele's Rule), Exact Conception / Ovulation Date, IVF Embryo Transfer (Blastocyst / Day 5 Transfer), Year (e.g. e.g. 2026), Month (1 - 12) (e.g. e.g. 1 (January)), Day of Month (1 - 31) (e.g. e.g. 15), and Average Menstrual Cycle Length (Days) [for LMP] (e.g. e.g. 28 (Normal range: 21 - 35 days)). Results update instantly — no submit button needed. Use metric or imperial units as labeled. For the most accurate result, measure height and weight in the morning without shoes.

The Formula

Obstetric Due Date (EDD) & Gestational Age equations:

Naegele's Rule (28-day cycle): EDD = LMP + 1 Year - 3 Months + 7 Days = LMP + 280 Days

Parikh's Formula (Adjusted Cycle): EDD = LMP + 280 Days + (Cycle Length - 28)

Conception Date Method: EDD = Conception Date + 266 Days (38 Weeks)

IVF Blastocyst (Day 5) Method: EDD = Transfer Date + 261 Days

Gestational Age (Weeks) = (Current Date - LMP Date) / (7 Days)

Worked Example

For example, entering a Last Menstrual Period of January 15, 2026 with a 30-day cycle: Standard 280 days plus 2 additional cycle days equals 282 days. The calculated Estimated Due Date is October 24, 2026. If today is May 1, 2026, the gestational age is exactly 15 weeks and 1 day (Second Trimester).

Understanding Your Result

Naegele's Rule (LMP + 280 days) gives the estimated due date — only about 5% of babies arrive on their exact due date. The normal delivery window is 37–42 weeks gestation. Babies born 37–38+6 weeks are "early term" and may have higher risk of respiratory and feeding difficulties than babies born 39–40 weeks. Ovulation typically occurs 12–16 days before the next menstrual period, not necessarily on day 14 — cycle length variation shifts this window significantly.