Where Does Fertilization Occur?

You might not be looking to get pregnant right now, but if you’re curious how that actually happens when the time comes, you’re in the right place. It’s incredibly important to understand the actual process of how pregnancy occurs because after all, knowledge is power, right?  

But before we jump right into how an egg becomes fertilized, let’s go over the process that happens before fertilization.

The process of fertilization

It all starts with an egg

Females are born with two ovaries that hold millions of immature eggs which all have hopes of one day becoming an embryo, also known as a zygote. Every cycle, a subset of these eggs are ‘recruited’ to the surface of the ovaries and a single egg is selected to grow and ovulate. 

At the time of ovulation during the menstrual cycle, a mature egg leaves the ovary and travels through the fallopian tube where it crosses its fingers (metaphorically) that it will meet its potential match. That match being sperm, of course.

Don’t forget the sperm

Unfortunately, it’s not such an easy task for the sperm to get to the egg. This is a part of natural selection, so only the very best will survive. After ejaculation occurs, the sperm begins to travel along the female genital tract with the end goal being to meet the egg.

As we said, this trip isn’t a simple one. As the various sperm make their way to the egg, they’ll be pushed forward with the help of cervical mucus, but the acidic genital environment and immune cells of the woman can cause some sperm to die. It is up to the strongest spermatozoa to make it to the crown and lining of the egg.

When the sperm meets the egg in the fallopian tubes

This is where the real magic happens. Most sperm try to penetrate the egg, but not all make it through. The strongest sperm will make their way through the ovum, but only the fastest will penetrate the egg and actually fertilize.

When the strongest sperm meets the ovum an acrosomal reaction begins which will release enzymes that will eventually dissolve the outer layer of the egg.

They form an embryo

Now that the egg has found a match, within the next few hours, genetic material from the sperm and egg begin to integrate to create a zygote. This zygote holds a double set of chromosomes which is considered a single-cell embryo.

The embryo travels to the uterus

Now that the zygote has formed, it will then begin to make its way through the fallopian tube and into the uterus. Luckily, the lining of the fallopian tube is  covered in cilia which helps direct the embryo to the uterine cavity. 

Embryo division

Not only does the zygote have an important job of traveling to the uterus but it also begins to divide numerous times in order to become a multicellular embryo which is also known as a blastocyst. The division period starts immediately upon fertilization and can last for up to 3 days. Beyond day 3, the embryo begins to differentiate into two cell types: cells that will one day become the baby, and those that will turn into placenta. This is called a blastocyst stage embryo (from day 5 to 7 post-fertilization).

Implantation in the uterus

When the blastocyst has been established, the embryo must attach to the uterine wall which typically occurs about 6 days after fertilization and is completed around day 9 or 10. Pregnancy is truly achieved once this attachment occurs. 

The adherence to the wall is a crucial step as this is where the embryo will receive its nourishment and oxygen from the mother throughout the development of the pregnancy.

How is the sex of the baby determined?

While you may not find out for a few weeks what the sex of your baby is, you may be surprised to know that this is one of the first defining factors of the embryo during natural fertilization. This is thanks to that one sperm that won the race. 

A zygote contains 46 chromosomes, half of which come from the mother and the other half from the father. The zygote receives the 46 chromosomes due to the fact that each sperm cell holds 23 chromosomes, as well as the egg nucleus.

However, the egg and sperm carry different types of chromosomes. While an egg cell contains only X chromosomes, a sperm cell contains either an X or Y chromosome. When both cells split off to create their own zygote, you may end up with a girl by obtaining an X from the egg and an X from the sperm (X+X), OR on the contrary, a Y carrying sperm may merge with the egg creating a boy (X+Y).

Where does fertilization occur during IVF, IUI, ICSI, and PICSI?

If you’re having trouble getting pregnant naturally, your doctor may suggest looking into In Vitro Fertilization (IVF), Intrauterine Insemination (IUI), or Intracytoplasmic Sperm Injection (ICSI) as other methods of conception. However, instead of an egg becoming fertilized within a woman’s body, it occurs in a lab setting. 

With the use of healthy mature eggs and sperm to fertilize them, fertilization can occur inside a culture dish. Mind blowing, right? For those that struggle with natural fertilization, IVF, IUI, and ICSI are possible routes to explore. 

Throughout the years, these processes have become significantly more advanced and can be a successful way for one to become pregnant. Each process is a little bit different, so it’s important to understand the difference between IUI vs. IVF vs. ICSI.

Learn more about more accessible — even free — egg freezing

Cofertility is a fertility ecosystem that enables women to freeze their eggs for free when they donate half of the eggs to a family that otherwise can’t conceive, providing support and education for everyone involved along the way. Learn more about Freeze by Co!