Conception Date: How to Calculate When You Conceived
Learn the science behind conception date calculation, including how ovulation timing, sperm lifespan, and fertility windows affect when conception actually occurs.
🤰 Conception Age Calculator
Calculate when conception occurred and find out exactly how far along the pregnancy is. Enter your due date or last menstrual period to get started.
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📊 Your Results
—Conception Date
—Fetal Age
—Gestational Age
—Trimester
—Due Date
—Weeks Until Due
💡 How Conception Age Is Calculated
📅 Due Date Method
If you know your expected due date, conception likely occurred approximately 266 days (38 weeks) before that date. This method works backwards from the due date to estimate the day of conception.
🩸 LMP Method
Using the first day of your last menstrual period (LMP), conception typically occurs about 14 days later. The due date is calculated as LMP + 280 days (40 weeks), known as Naegele's rule.
🧬 Fetal Age vs Gestational Age
Fetal age (or conception age) is the actual time since conception — typically 38 weeks. Gestational age counts from the first day of LMP — typically 40 weeks. Gestational age is about 2 weeks ahead of fetal age.
📋 Pregnancy Trimesters at a Glance
Trimesters are based on gestational age (weeks since LMP). Conception/fetal age is approximately 2 weeks less.
| Trimester | Gestational Weeks | Fetal Weeks | Key Developments |
|---|---|---|---|
| First Trimester 🍼 | Weeks 0–13 | Weeks 0–11 | Major organs form, heartbeat detectable, morning sickness common |
| Second Trimester 🌟 | Weeks 14–26 | Weeks 12–24 | Baby grows rapidly, movement felt, gender can be determined |
| Third Trimester 👶 | Weeks 27–40 | Weeks 25–38 | Baby gains weight, lungs mature, baby moves into birth position |
🍼 First Trimester (Gestational Weeks 0–13)
This is the earliest stage of pregnancy. Conception occurs around week 2 of gestational age. Major organs including the heart, brain, and lungs begin to form. The embryo becomes a fetus by week 11. Many mothers experience morning sickness and fatigue during this period.
🌟 Second Trimester (Gestational Weeks 14–26)
Often considered the easiest trimester. The baby grows rapidly, and movements (quickening) become noticeable. The gender can be determined via ultrasound. Many mothers find their energy returns, and the belly becomes visibly pregnant.
👶 Third Trimester (Gestational Weeks 27–40)
In the final stretch, the baby gains weight quickly and organs mature, especially the lungs. The baby typically moves into a head-down position for birth. Braxton Hicks contractions may begin. Full-term pregnancy is considered 39–40 weeks gestational age.