Your baby doesn’t magically — poof! — turn into an embryo immediately after conception. There’s 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.[1] It’s created when sperm meets an egg in one of the fallopian tubes. When the sperm mixes with the egg, you conceive, and the result is the zygote.
A zygote contains all the genetic information (DNA) that’s required to create a little human being. Half of that comes from the egg, while the other half comes from the sperm.
"The zygote is a single cell and contains the genetic blueprint to direct the next complex steps to become the embryo and fetus," says What to Expect Medical Review Board Member Aaron K. Styer, M.D., founding partner and medical director of the Colorado Center for Reproductive Medicine (CCRM) Boston in Newton, Massachusetts.
How does a zygote form?
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 thinner and more elastic than usual, allowing sperm to reach the uterus quickly.
Once those sperm show up, a few hundred to a thousand will try to penetrate into the egg. Only one lucky one will get in.
When it does, chemical changes in outer covering of the egg itself occur so that no other sperm can break through. And bingo: A zygote is formed.
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Sometimes your ovaries release more than one egg. If both become fertilized, then congratulations! You may be having twins, notes Dr. Styer.
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.
“Interestingly, eggs may be released at different times, separated by a few days,” notes Dr. Styer. “As a result, fraternal twins may differ in age by several days.”
You’re more likely to have fraternal twins if you have a family history of them.[2] Women with moms or sisters who have had these types of twins are about twice as likely to have twins 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.
“Your embryos are then grown in a sophisticated incubator that mimics the environment of the fallopian tubes,” says Dr. Styer.
Then at least one embryo is transferred into your uterus with the end goal that it will result in a successful pregnancy.
What happens when a zygote becomes an embryo?
It takes about five to six days for a zygote to transform into a blastocyst, which is 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 3 or 4 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.
“Although they contain the same genetic material and are the same sex, identical twins don’t always look the same,” says Dr. Styer. “There can be a difference in height, weight and hair color.”
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. 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.
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, they’ll combine to create the full 46 chromosomes (two sets of 23, one from each gamete) needed for a full human cell.
What happens when zygotes don't make it to the next stage?
Research suggests that an estimated 30% of all naturally occurring conceptions fail. While experts aren’t sure why a zygote may not progress to the next stage of development, they think it may be due to chromosomal abnormalities and other factors.
Often a woman may not know she’s experienced an early miscarriage (also called a chemical pregnancy) because any bleeding may take place during the time she’d typically have her period, masking the pregnancy loss.
However, new home pregnancy tests make it possible to find out about early pregnancies as soon as four days before an anticipated period.
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