Calculators

Health

When Did I Get Pregnant Exactly?

There are multiple ways to calculate pregnancy conception date. Each calculation results in an estimated date range when conception likely occurred. You can calculate conception date using:

• Pregnancy due date
• Last menstrual period (LMP) date
• Ultrasound results (gestational age)
• A birth date

It's important to note that all conception calculations are estimates because ovulation and fertility timing varies from person to person.

Calculate Conception Date Using Due Date

Your healthcare provider determines your due date. The formula for calculating conception date using due date is:

Conception Date = Due Date − 266 Days

Why 266 days? In a typical 28-day cycle, ovulation and conception occur about 14 days after the last period starts. Since a 40-week pregnancy is measured from the start of your last period, the actual time from conception to birth is about 2 weeks shorter: 40 weeks − 2 weeks = 38 weeks, or 266 days.

Example: Your healthcare provider determines your due date is August 9.

• Subtract 266 days from the due date: August 9 − 266 days = November 16
• The estimated conception date is November 16 of the previous year.

Calculate Conception Date Using LMP

This method accounts for your specific cycle length to figure out when conception likely happened.

If you have a regular cycle length, the formula for calculating conception date using LMP date is:

Conception Date = LMP + (Cycle Length − 14 days)

1. Find your cycle length which is number of days from the first day of one period to the day before your next period. If your cycle length varies use your average cycle length.
2. Subtract 14 days from your cycle length.
3. Add that number to the first day of your last period.

Why cycle length matters: While the average cycle is 28 days, normal cycles range from 21 to 35 days. Many women track their menstrual cycle for family planning. This is important because ovulation usually happens 14 days before your next period, regardless of cycle length. This means:

• 25-Day cycle: 25 - 14 = 11 → Ovulation around day 11
• 28-Day cycle: 28 - 14 = 18 → Ovulation around day 14
• 35-Day cycle: 35 - 14 = 21 → Ovulation around day 21

Example: You are pregnant and your last period started on April 24. What is your estimated conception date?

• You have an average cycle length of 27 days.
• Subtract 14 days from your cycle length: 27 − 14 = 13 days
• Add 13 days to the first day of your last period: April 24 + 13 days = May 7
• The estimated date of conception is May 7.

Calculate Conception Date Using Ultrasound and Gestational Age

Early pregnancy ultrasounds are the most accurate way to determine conception, especially when performed between 6 and 8 weeks. Ultrasounds find gestational age by measuring the crown-rump length, the length of the embryo from the top of its head to its bottom. Finding gestational age with this measurement is accurate within 5 to 7 days.

The formula for calculating conception date given ultrasound date and gestational age is:

Conception Date = Ultrasound Date − (Gestational Age − Ovulation Window)
Where Ovulation Window = Cycle Length − 14

1. Note your ultrasound date and gestational age (given in weeks and days).
2. Convert gestational age to days: ( Weeks × 7 ) + Remaining Days
3. Calculate ovulation window: Cycle Length − 14 Days
4. Subtract results of Step 3 from results of Step 2. This is the same as subtracting days to ovulation from gestational age in days.
5. Count backward that many days from your ultrasound date.
6. The final date is your estimated conception date.

Example: An ultrasound on January 15 shows a gestational age of 7 weeks 5 days. The person has a 30-day cycle.

• Find gestational age in days: ( 7 × 7 ) + 5 = 54 days
• Calculate ovulation window using cycle length: 30 − 14 = 16 days
• Subtract ovulation window from gestational age: 54 − 16 = 38 days
• Count back 38 days from the ultrasound: January 15 − 38 days = December 8
• The estimated conception date is December 8, plus or minus 5–7 days (so roughly December 1–15).

Calculate Conception Date Using an Actual Date of Birth

It takes an average of 38 weeks from fertilization of an egg to birth of a baby. This is the length of gestation in humans. If you include the first two weeks of the menstrual cycle before an egg is fertilized, like calculating due date from LMP, the typical pregnancy length is 40 weeks.

However in reality most full-term pregnancies can vary in length from 38 to 42 weeks. This is 40 weeks, plus or minus two weeks. So if you want to calculate conception date using an actual birth date, you need to consider that a full-term baby could have been conceived 38–42 weeks before birth.

The formula for calculating conception date using an actual birth date is:

Conception Date = Date of Birth − 266 Days, plus or minus 2 weeks

Why 266 days? The average full-term gestation in humans is 38 weeks. 38 × 7 = 266 days. Let's consider an example, calculating conception date from an actual date of birth.

Example: Your parents were traveling around Europe the summer they conceived your brother. You'd like to calculate his estimated date of conception to see where your parents might have been visiting around that time. Your brother's date of birth is April 2, 1990.

• Subtract 266 days from your brother's birth date
  Birth date - 266 days = Conception date
  April 2, 1990 − 266 days = July 10, 1989
• You're not sure if your brother's delivery was early, late, or "on time" so the possible window of his conception is July 10, 1989 plus or minus 2 weeks.
• July 10 minus 14 days = June 26
• July 10 plus 14 days = July 24
• Your brother was likely conceived between June 26 and July 24, 1989.

Ovulation, Fertilization & Conception

The fertile window is the time each month when a person can get pregnant. It includes the five days before ovulation plus ovulation day itself.

Ovulation is the day the ovaries release an egg that can be fertilized.

Conception occurs when sperm fertilizes the egg during or shortly after ovulation. The fertilized egg then travels to the uterus and implants into the uterine lining. This is when pregnancy officially begins.

Frequently Asked Questions