How Pregnancy Calculator computes the fetal age

Q: Why doctors typically calculate the fetal age from the onset of your last menstrual cycle?

A: The conception date is approximately 14 days after the onset of the menstrual cycle. However, the exact conception date is hard to tell because:

- The menstrual cycle is often not 100% regular.
- Ovum lives for approximately 3 days.
- Sperm lives for about 2 days.

Because of the uncertainly of the exact conception date, obstetricians prefer to calculate the fetal age from the first day of the last menstrual cycle.

This is inaccurate because the fetus cannot be 2 weeks old even before it has been conceived!

...but this is more practical because the onset of the menstrual cycle is more certain than the conception date.

The Pregnancy Calculator gives you both: 1. the fetal age, and 2. how far along you are in your pregnancy (often referred to as "fetal age" by the obstetricians).

Pregnancy Calculator

Use the Pregnancy Calculator to:

Determine the due date
Calculate the likely conception date
Calculate the end of the first and second trimesters
Find out for how many weeks have you been pregnant
Estimate the fetal age
Calculate how many weeks you will be pregnant on a given date
Get an idea for how long you'll be active
Calculate on which date you will be pregnant the specified number of weeks, and
Determine the likely conception date when you know the birthday or due date.

Gestational age (or fetal age) is the time that a fetus has been developing in the mother's womb from the date of conception. Our Pregnancy Calculator gives you both, the fetal age and how far along you are in your pregnancy.

For full explanation, see How Pregnancy Calculator computes the fetal age.

In short, why do they differ:

Fetal age (from the estimated conception date)
Calculating the gestational age from the conception date is more accurate. It is the preferred way of the medical textbooks.
How far along you are in your pregnancy (from the first day of the last menstrual cycle)
Calculating the gestational age from the from the first day of the last menstrual cycle is more practical. It is the preferred way of obstetricians.

Pregnancy Calculator
When was the first day of your last menstrual period?

Number of days in your menstrual cycle

Likely conception date:
End of first trimester (12 weeks):
End of second trimester (27 weeks):
Your due date is (40 weeks):
(1) Fetal age (from the conception date):
(2) How far along you are in your pregnancy:
How many weeks will you be pregnant on a given date?


Number of weeks you will be pregnant for:
On which date will you be pregnant for the given number of weeks?
The number of weeks:

Calculated date:
Given the birthday or due date, calculate likely conception date


Calculated conception date:
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How to use the Pregnancy Calculator

Enter the date of your last menstrual period and the number of days in your menstrual cycle into the pregnancy calculator, and click on the Calculate button. Other calculations are optional - use them when needed.

On average, there are 28 days in a menstrual cycle, but it can last from as little as 22 to as many as 44 days. The Pregnancy Calculator takes your menstrual cycle into account when calculating the due date and the conception date.

Also, the Pregnancy Calculator needs the number of days in your menstrual cycle when calculating the likely conception date from the given birthday or the due date. It's a very useful tool as perhaps you were busy playing bingo, or with work commitments, or just have a busy life and you aren't sure of these things.

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