How a Zygote Forms and Becomes an Embryo

Your baby doesn’t magically poof! turn into an embryo immediately after conception. It goes through a whole process that you probably learned about (and possibly forgot) in your high school biology class. Central to this all is a zygote, which is what’s formed after a sperm and egg have a little rendezvous. Here’s what happens.

What is a zygote?

A zygote is a fertilized egg. It’s created when sperm meets an egg in one of the fallopian tubes. When the sperm enters the egg, you conceive and the result is the zygote. It contains all the genetic information (DNA) that’s required to create a little human being. Half of that comes from the egg, and the other half comes from the sperm.  

What is the difference between a zygote and a gamete?

Here’s the 411 on a gamete versus a zygote: Gametes are sex cells. They carry only one set of each chromosome, making them what's called haploid cells. Female gametes are ova or egg cells, and male gametes are sperm. 

When they join together to form a zygote — aha! — they’ll combine to create the full 46 chromosomes (two sets of 23, one from each gamete) needed for a full human cell.

How is a zygote formed?

It’s time for a little sex-ed primer: When ovaries release an egg during ovulation, it’s swept into one of your fallopian tubes, where it waits for sperm to arrive. To encourage this, the mucus in the cervix becomes more liquid and elastic, allowing sperm to reach the uterus quickly. 

Once those sperm show up, a few hundred will try to penetrate into the egg. Only one lucky one will get in. Once it does, chemical changes in the egg itself occur so that no other sperm can break through. Bingo! A zygote is formed.

Sometimes, your ovaries release more than one egg. If both become fertilized, then congrats! You’ve got twins. These are known as fraternal twins, which means that while they’ll share the same age and birthday (and mom’s uterus), they’ll look and act differently because they’re made from two different eggs fertilized by two separate sperm. 

You’re more likely to have fraternal twins if you have a family history of them. Women with moms or sisters who have had these types of twins are about twice as likely to have them themselves. 

If you have trouble conceiving and use artificial insemination — when sperm is placed inside your reproductive tract during ovulation — it’s the same concept, only done a bit differently. In intrauterine insemination (IUI), a doctor uses a thin catheter to inject sperm directly into your uterus to increase the chances that sperm will meet the egg. 

During in vitro fertilization (IVF), your eggs are fertilized by sperm in a laboratory at a fertility clinic. Then at least one embryo is transferred into your uterus with the hope that it will spark at least one (and possibly more!) babies. 

How long does it take a zygote to become an embryo?

It takes about five to six days for a zygote to transform into a blastocyst (a microscopic ball of cells) and then into an embryo.

Within hours after sperm meets egg, the zygote divides and then continues to divide (and divide). Within days, it’s turned into a blastocyst that's around one-fifth the size of a period. 

The blastocyst now begins its big journey from your fallopian tube to your uterus. Very rarely — about three or four out of 1,000 births — the zygote splits in half, leading to two embryos. Since the original egg was fertilized by one sperm, the genetic material in both embryos is identical, resulting in (you guessed it) identical twins. 

It’s estimated only about one-third of all zygotes — or fewer — survive the first four weeks of pregnancy. Thankfully, the ones who do are usually sturdy enough to make it through.

Once the blastocyst reaches your uterus and attaches to the lining, this little ball of cells does another great divide. Half becomes your baby, while the other half forms the placenta, which is how she’ll eat and drink while snuggled in your uterus. 

Congrats! Your little zygote has now officially turned into an embryo. About eight weeks after fertilization (roughly week 10 of pregnancy) the embryo will turn into what’s known as a fetus. From there, it's only about 30 more weeks until you'll hold a little bundle of joy in your arms.