Pregnancy Conception Calculator

The pregnancy conception calculator estimates a likely conception date and possible conception range from a due date, last menstrual period with cycle length, or ultrasound dating. It also returns a calculated due date. Because conception timing is medically and personally sensitive, treat the result as educational date arithmetic, not proof of ovulation, intercourse timing, paternity, or pregnancy health.

What the calculator can and cannot tell you

Pregnancy dating usually begins before conception, with gestational age counted from the first day of the last menstrual period or an equivalent dating start. Conception generally occurs around ovulation, but ovulation varies, sperm can survive for several days, and fertilization is not directly visible on a calendar. For that reason, the calculator gives one estimated conception date and a range from five days before to one day after that estimate.

Use the gestational age calculator if you want current pregnancy age in weeks and days. Use the pregnancy due date calculator for due-date-centered planning, or the reverse due date calculator when you are starting from a target delivery date. For ordinary birthdays after birth, the age calculator uses different calendar-age logic.

Method and calendar rules

The calculator has three methods. In due date mode, the calculation parses the entered due date and subtracts 38 weeks. Since 38 times 7 is 266, the formula is:

$\text{estimated conception date} = \text{due date} - 266\ \text{days}$

The calculated due date remains the date you entered.

In last period mode, the calculation parses the last period start date and cycle length. It estimates conception as cycle length minus 14 days after the period began:

$\text{estimated conception date} = \text{last period date} + (\text{cycle length} - 14)\ \text{days}$

It also calculates a due date by adjusting the standard 40-week count for cycle length:

$\text{calculated due date} = \text{last period date} + (\text{cycle length} - 28 + 280)\ \text{days}$

In ultrasound mode, the reported gestational age is converted to total days:

$\text{scan gestational days} = 7 \times \text{weeks} + \text{days}$

the calculation estimates conception by subtracting gestational age minus 14 days from the scan date:

$\text{estimated conception date} = \text{scan date} - (\text{scan gestational days} - 14)\ \text{days}$

The due date is the scan date plus the remaining days to 280 gestational days:

$\text{calculated due date} = \text{scan date} + (280 - \text{scan gestational days})\ \text{days}$

For every method, the possible range is fixed around the estimated conception date:

$\text{range start} = \text{estimated conception date} - 5\ \text{days}$

$\text{range end} = \text{estimated conception date} + 1\ \text{day}$

The result item says the range length is 7 days.

Example: estimating a conception date

Choose Due date and enter 2026-10-08. The calculator subtracts 266 days. That lands on January 15, 2026, so the primary result is an estimated conception date of January 15. The possible range starts five days earlier, January 10, 2026, and ends one day later, January 16, 2026. The due date item remains October 8, 2026, and the range length item displays 7 days.

For a last-period example, enter January 1, 2026 and a 28 day cycle. Cycle length minus 14 equals 14, so the estimated conception date is January 15, 2026. The due date formula adds 28 - 28 + 280 days to January 1, also giving October 8, 2026. If the cycle length is 32 days, conception shifts four days later and the due date shifts four days later because the method assumes later ovulation.

For an ultrasound example, a scan on March 1, 2026 showing 8 weeks, 3 days has a gestational age of 59 days. The calculator subtracts 59 - 14, or 45 days, from the scan date to estimate conception. It adds 280 - 59, or 221 days, to estimate the due date.

Interpreting the conception range

The range is deliberately broader than a single date. It reflects sperm survival before ovulation and uncertainty in when ovulation, fertilization, and implantation occurred. It does not prove that intercourse happened on any date in the range, and it does not exclude possibilities outside the range when cycles are irregular or dating is uncertain.

ACOG guidance on due-date estimation focuses on the best obstetric estimate rather than retrospective certainty about conception. MedlinePlus describes gestational age as the common way pregnancy age is measured. Those clinical conventions are useful for prenatal care, but they are not forensic tools. If the result affects relationships, legal questions, or medical care, seek appropriate professional advice.

Edge cases and common mistakes

The last-period method assumes a cycle length from the calculator field and an ovulation timing of cycle length minus 14 days. That may be inaccurate for irregular cycles, polycystic ovary syndrome, recent pregnancy, breastfeeding, emergency contraception, fertility medication, or assisted reproduction. In IVF or IUI contexts, clinic-provided dates should take priority.

The ultrasound method depends on the scan’s reported gestational age. Early ultrasounds tend to be used for dating more often than later growth scans, but your clinician decides whether a scan changes the official due date. The current the calculation function accepts very early ultrasound ages; if a scan age is less than 2 weeks, the formula can place the estimated conception date after the scan date. That is a formula edge case to discuss clinically rather than a meaningful conception estimate.

Calendar and date boundaries

This is Gregorian calendar-day arithmetic: leap days and varying month lengths are included when counting backward or forward. Dates have no time-of-day, time-zone, or daylight-saving offset, so crossing midnight in a particular location is not modeled. Date boundaries describe an estimated window and cannot identify the exact instant of conception or intercourse.

Sources

• ACOG, Methods for Estimating the Due Date — clinical guidance on assigning and revising estimated due dates.
• MedlinePlus, Gestational age — patient-facing definition of gestational age and pregnancy dating language.
• ACOG, How Your Fetus Grows During Pregnancy — general patient education on pregnancy development by week.