Pregnancy conception calculator

This pregnancy conception calculator helps you estimate the likely date of conception and your fertile window based on your last menstrual period, due date, or ultrasound date. While exact dates can vary, this tool provides a helpful range to understand when pregnancy may have begun.

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Understanding how pregnancy begins

It’s natural to wonder when pregnancy actually started—especially for those trying to understand the timing of conception. Medically, pregnancy is dated from the first day of your last menstrual period (LMP), not from the actual date of conception. This means that gestational age usually includes about two weeks before fertilization occurs.

Although it’s difficult to identify the precise moment of conception, this calculator provides an estimated range based on key dates such as your LMP, due date, or ultrasound results.

Ways to estimate when conception occurred

Based on your last period

For many with regular menstrual cycles, ovulation—and therefore the opportunity for conception—typically happens around 11 to 21 days after the LMP. Since sperm can live inside the body for several days, intercourse leading to conception may occur within a broad window. However, if periods are irregular or the LMP is uncertain, this estimate becomes less reliable.

Using an expected due date

If your healthcare provider has given you a due date, you can work backwards to estimate conception. In general, subtracting 266 days from the due date gives a rough idea of when fertilization likely occurred.

With the help of ultrasound

First-trimester ultrasounds are often the most accurate method for dating a pregnancy. They measure the size of the embryo or fetus to estimate how far along the pregnancy is. This can be especially helpful when the LMP is unclear or cycles are irregular.

What actually happens during conception

There’s often confusion about the terms “fertilization” and “conception.” Here’s how they differ in medical terms:

• Fertilization happens when a sperm cell joins with an egg in the fallopian tube.
• Conception is considered complete only after the fertilized egg travels to the uterus and successfully implants into the uterine lining.

While fertilization occurs shortly after ovulation, implantation usually takes an additional 6–10 days. Only once implantation happens is pregnancy officially established.

Why conception dates are only estimates

There are several reasons why it’s hard to determine the exact day of conception:

• Ovulation doesn’t always follow a predictable schedule—even in those with regular cycles.
• Sperm can survive for up to five days, meaning intercourse before ovulation can still result in pregnancy.
• Implantation happens several days after fertilization, adding further uncertainty.
• Medical dating typically starts with the LMP, not fertilization.

Because of all this, conception calculators aim to provide a range of likely dates, not a single pinpointed moment. For the most accurate assessment, especially in early pregnancy, consult a healthcare provider.