Gestational age vs. fetal age: what's the difference?

One of the most common sources of confusion in pregnancy is that there are two different ways to count the weeks, and they're about two weeks apart. Getting them straight explains why you can be told you're “six weeks pregnant” only a few weeks after conception.

Two clocks

Gestational age is counted from the first day of the last menstrual period (LMP). Fetal age (also called conceptional or embryonic age) is counted from conception. Because conception happens roughly two weeks after the last period on a typical cycle, gestational age is about two weeks larger than fetal age throughout the pregnancy.

So at “eight weeks pregnant” (gestational age), the embryo has actually been developing for about six weeks (fetal age). Both numbers are correct; they're just measured from different starting lines.

Which one do clinicians use?

Almost always gestational age. Prenatal care, screening windows, growth charts, and the definition of “term” are all keyed to gestational age counted from the LMP. When you read that a pregnancy is “24 weeks,” that's gestational age. Our calculator and every standard pregnancy wheel report gestational age for this reason.

Fetal age shows up mainly in embryology and in fertility treatment, where the exact day of fertilisation is known. That's why IVF dating is unusually precise — see IVF due date calculation.

Why count from the last period at all?

Because it's the date people can actually pin down. Most don't know the day they conceived, but many can recall when their last period started. Dating from the LMP gives a consistent, shared reference that works across the whole population, even if it's an approximation for any one pregnancy. The trade-offs of that approximation are covered in Naegele's rule.

How this maps onto the trimesters

All the familiar landmarks use gestational age: the first trimester runs to about 13 weeks, the second from 14 to 27, and the third from 28 weeks to birth. Viability and term are defined the same way. For the full map, see pregnancy trimesters and milestones.

The quick version

• Gestational age = weeks since the last period. This is the number you'll be quoted.
• Fetal age = weeks since conception, about two weeks less.
• Ultrasound reports gestational age, so it lines up with clinical use.

When in doubt, assume any “weeks pregnant” figure means gestational age. If someone gives you a conception date instead, the conception mode converts it for you.