Embryo vs Fetus: What’s the Difference?

Depending on how far you are through your pregnancy, your doctor might use different words to refer to your growing baby. Sometimes you might hear that you’ve got an “embryo” in there, and at other times it’s a “fetus.” So, embryo vs fetus (or even embryo vs fetus vs zygote!): what’s the difference?

Read on to find out.

In this article 📝

When does an embryo become a fetus?

The difference between an embryo and a fetus is all in the timing.

For most of your first trimester (from week 5 to week 10), your baby is known—in medical speak—as an embryo.

So it’s a special moment when, at week 11, they “graduate” to full fetus status.

Let’s take a look at the timeline of your pregnancy as your baby makes the journey from embryo to fetus.

Embryo vs fetus: Your baby’s journey of development

Week 1: Your period

The first day of your last menstrual period is usually counted as the beginning of your pregnancy (even though you’re not technically pregnant yet!).

Week 1 sees your uterus shedding its lining, which will then start to regrow, ready to receive a fertilized egg.

Week 2: Preparing for ovulation

At week 2, your body is gearing up to release an egg.

You’ll also produce more cervical mucus for sperm to swim through to reach the egg when it arrives.

Week 3: Ovulation and fertilization

The big day has arrived!

At the start of week 3 (or thereabouts, depending on the length of your cycle), your ovaries release an egg.

If the egg is fertilized by a sperm cell, it becomes known as a zygote.

It’s a tiny single-celled organism, but full of potential…

So let’s put aside embryo vs fetus for a second and consider zygote vs embryo.

When does a zygote become an embryo?

Actually, not until week 5 of your pregnancy.

That little zygote has to go through a whole other stage of development first.

By day 5 or 6 after fertilization, the zygote has divided into a ball of cells called a blastocyst.

The blastocyst continues to divide to produce more and more cells as it makes its way down the fallopian tube towards your uterus.

Week 4: Implantation

Around week 4, the blastocyst reaches your uterus and burrows down snugly into your uterine lining.

At this point, it’s ready to begin developing into both an embryo and the placenta (the organ that will nourish your baby during their time in your uterus).

Implantation is also when your body starts to produce the hormone hCG (human chorionic gonadotropin), which a home pregnancy test can detect.

Week 5 to 10: Embryonic stage

At week 5, your baby is ready to move on from being a blastocyst and embrace their new identity as an embryo.

So, what is an embryo, exactly? How is it different from a blastocyst?

What is the embryo stage?

Your baby starts to be known as an embryo once the amniotic sac has developed.

That’s the membrane filled with fluid that will hold and protect your baby until the moment your waters break, around the time you go into labor.

The embryo stage lasts until week 10 of your pregnancy.

During this time, all your baby’s essential organs and bodily structures are formed: heart, brain, arms, legs—even toes and eyelids!

But everything is still extraordinarily tiny.

By the end of the embryonic stage, your baby will be around 2 inches long.

Week 11 to 40 (ish): Fetal stage

Your baby starts to be called a fetus from week 11 of your pregnancy.

The main difference between embryo and fetus?

Well, by the time your baby becomes a fetus, their essential organs and bodily systems are already formed, to a great extent, and now all they need to do is keep growing and developing.

Their bones will become stronger, they’ll put on weight, and you’ll start to feel them move around in your uterus.

And, before you know it, that fetus will be ready to greet the world as a fully-fledged baby! 👶

You might also be interested in: Your Pregnancy Week-by-Week Guide When Does a Fetus Develop a Brain? When Does the Placenta Form? When Do You Find Out the Gender of a Baby? How Far Along am I? How to Work it Out Your Fetus Weight Percentile Calculator Understanding Fetal Brain Development