Fertilized Egg: What Happens to It?

Just like you learned in school, all babies start off as a fertilized egg – but if that’s where your memory of the process starts and finishes, then you’re certainly not alone! But if you’re trying to conceive, then you’re probably going to be hearing a lot of about fertilized eggs. How exactly do you get from there to a baby? Let’s run through the process of fertilization through to cell division and specialization that occurs before implantation into the uterine lining.

What is a fertilized egg?

fertilized egg

The journey to a fertilized egg begins when the woman’s body releases an egg cell (oocyte) from a mature follicle at ovulation. The egg is now ready to be fertilized. When sperm enter the female reproductive tract at intercourse, they move towards the egg.

One sperm cell is able to make through and combine with the egg, and the egg cell is officially fertilized at this point. This is when the male sperm and the female egg come together to form a single cell containing chromosomes from both Mom and Dad. This fertilized egg is called a zygote. It’s a just tiny cell – just 0.1mm across! – but the zygote has all the genetic information needed to build a human.

How does a fertilized egg make a baby?

fertilized egg

When the egg and sperm combine to form a fertilized egg, it doesn’t immediately implant into the uterus. First, the zygote has to undergo many rounds of cell division until it reaches the state where it’s able to implant. The cell splits over and over in a process called cleavage. This differs from other kinds of cell division in that it increases the number of cells, but not the total mass: the cells split, but the fertilized egg remains the same size. This cleavage forms a mass of cells called a morula.

Countdown to implantation: Fertilized egg, morula, or blastocyst?

fertilized egg

It takes time before the fertilized egg is able to implant into the uterus. During this time, it undergoes more and more rounds of cell division and turns into a mass of 200 to 300 cells! Let’s follow its journey along the fallopian tubes.

1 day to 2 days after fertilization

The cell splits from one egg into two. This marks the start of the germinal stage of development. The fertilized egg begins to make its way slowly through the fallopian tubes towards the uterus. (Incidentally, it’s also possible for the cell to split entirely at this stage. If that happens, you end up with identical twins!)

3 to 4 days after fertilization

After 4 or more rounds of cell division, the cluster of cells is known as a morula – named the Latin word for a mulberry. It appears as a cluster of cells which looks like a mulberry.

5 to 6 days after fertilization

Until this stage, the fertilized egg has undergone several rounds of cell cleavage, and its cells have divided into the same size. At this stage, the cells begin to take on different, more specialized roles.

With a thin, membrane-like layer of cells called the trophoblast on the outside, and a cluster of cells called the inner cell mass (or embryoblast) on the inside, this entire structure is now called a blastocyst. The trophoblast will become a part of the placenta, while the inner cell mass will eventually become the baby.

6 to 7 days after fertilization

While continuing to undergo cell division, the blastocyst arrives in the uterus. It will float about for a bit longer before finally implanting into the endometrium lining.

Fetal development begins from implantation

pregnant belly ultrasound

After ovulation, the body makes some adjustments to prepare for a possible pregnancy. The body releases the hormone progesterone, thickening and softening the lining of the womb in preparation for the implantation of a blastocyst.

On the other hand, while the blastocyst is floating about in the uterus, the zona pellucida (a layer of cells which covers the trophoblast) disappears, allowing the blastocyst to attach to the egg. After about two days of floating in the uterus, the blastocyst attaches to uterine lining. This is the final step: implantation.

Pregnancy all starts with a fertilized egg

The steps towards implantation all happen before you even realize that you’re pregnant, but your body is already undergoing some significant changes. Even at this early stage, all the necessary ingredients for making a person are in place! It might be a bit hard to get your head around some of the biology at first, but the more you learn about the changes taking place in your body at each step of the way, the more you can appreciate the mysterious and miraculous process of getting pregnant.