Pregnancy Due Date Calculator - LMP, Conception & IVF Transfer

Pregnancy Due Date Calculator – LMP, Conception & IVF Guide

Finding out you are pregnant is an exciting milestone. One of the first questions expecting parents ask is: "When is the baby due?" An Estimated Due Date (EDD) marks the estimated time when a full-term pregnancy reaches 40 weeks (280 days). This calculator supports three clinical calculation modes: Last Menstrual Period (LMP) mode using Naegele's Rule adjusted for your average cycle length, Conception Date mode based on the known date of ovulation or conception, and IVF Embryo Transfer mode based on a 3-day or 5-day IVF transfer date.

Understanding the 3 Calculation Modes

Last Menstrual Period (LMP) mode dates most pregnancies from the first day of the last menstrual period, since most patients know when their period started even if exact ovulation is hard to pinpoint:

EDD = LMP Date + 280 days + (Cycle Length - 28 days)

Standard Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your average cycle is 32 days, ovulation occurred approximately 4 days later, so 4 extra days are added to your due date.

Conception Date mode, for patients who know their exact ovulation or conception date (e.g. through ovulation tracking): EDD = Conception Date + 266 days, since fetal development takes approximately 266 days from actual conception.

IVF Embryo Transfer mode, where the embryo's exact age at transfer is precisely known: a 5-day blastocyst transfer uses EDD = Transfer Date + 261 days, while a 3-day cleavage transfer uses EDD = Transfer Date + 263 days.

Gestational Age vs. Fetal Age

Gestational age is measured in weeks and days from the first day of your LMP — when your doctor says you are "8 weeks pregnant," the embryo has actually been developing for about 6 weeks. Fetal age is the actual age of the developing fetus, measured from conception (2 weeks less than gestational age).

Trimester Breakdown

Pregnancy is divided into three trimesters: the First Trimester spans weeks 1 through 12 (early development and organ formation), the Second Trimester spans weeks 13 through 26 (rapid growth and baby movements), and the Third Trimester spans weeks 27 through 40 (final maturation and preparation for birth).

Worked Examples

Example 1: LMP Mode with a Standard 28-Day Cycle

LMP = January 1, 2026, cycle length = 28 days. Applying Naegele's Rule: Jan 1, 2026 + 280 + (28 - 28) = Jan 1, 2026 + 280 days = October 8, 2026.

Example 2: LMP Mode with a Longer 32-Day Cycle

LMP = January 1, 2026, cycle length = 32 days. Jan 1, 2026 + 280 + (32 - 28) = Jan 1, 2026 + 284 days = October 12, 2026 — 4 days later than the standard-cycle example, reflecting the later expected ovulation.

Example 3: IVF 5-Day Blastocyst Transfer

Transfer date = February 1, 2026 (5-day blastocyst). Feb 1, 2026 + 261 days = October 20, 2026. A 3-day cleavage transfer on the same date would instead add 263 days, landing on October 22, 2026 — 2 days later, matching the embryo's 2-day-younger age at transfer.

Why Due Dates Can Change

Only about 4% of babies are born on their exact estimated due date. Most healthy deliveries occur between 37 weeks and 42 weeks. Your healthcare provider may adjust your official due date after performing a first-trimester dating ultrasound, which measures the baby's crown-rump length (CRL) for high precision.

Frequently Asked Questions

What does an Estimated Due Date (EDD) mean?

An Estimated Due Date (EDD) is a medical estimate marking 40 weeks (280 days) from the first day of your last menstrual period. Only about 4% of babies are born on their exact due date; most arrive within 2 weeks before or after.

What is the difference between Gestational Age and Fetal Age?

Gestational age measures pregnancy progress starting from the first day of your last menstrual period (LMP). Fetal age measures actual developmental age starting from conception (typically 2 weeks after LMP).

How are pregnancy trimesters divided?

The First Trimester spans from week 1 to the end of week 12. The Second Trimester spans from week 13 to the end of week 26. The Third Trimester spans from week 27 until delivery.

Why do 5-day and 3-day IVF transfers use different day offsets?

A 5-day (blastocyst) transfer starts from an embryo that is already 5 days past fertilization, so only 261 more days are needed to reach the 266-day post-conception due date. A 3-day (cleavage-stage) transfer starts from an embryo that is only 3 days past fertilization, so 263 more days are needed — 2 more days than the blastocyst offset, exactly matching the 2-day difference in embryo age at transfer.