A female is born with all of the eggs she will ever have in her lifetime. One egg matures every month and is either fertilized or discarded through menstration, which starts during puberty. This happens until a woman no longer has periods, around the age of 45.

From here, the cells in the zygote multiply, forming the genetic makeup of an individual, based on a mix of chromosomes from the mother and father’s genes.

1

2

3An egg starts out in the ovaries and travels to the fallopian tubes and then to the uterus where it would be fertilized.

When the egg is fertilized, it becomes what is called a zygote.

BABYFORMATION

During which time frame that the splitting occurs will determine what type of identical twins they will become and, how they implant into the uterine lining.

3

1 An egg starts out in the ovaries and travels to the fallopian tubes and then to the uterus where it would be fertilized. Since itbegins with one egg, these arereferred to as monozygotic twins.

2 When the egg is fertilized, it becomes a zygote and the cellsstart to split.

The formation of identical twins starts out the same way that a single baby starts out.

IDENTICAL TWIN FORMATION

Zygote

Cell

1 In this case, the mother releases two eggs during ovulation, which is why fraternal twins are referred to as dizygotic twins.

2 Each egg is fertilized by two separate sperm.This is why fraternal twins don’t always look exactly alike, and are not always the same sex.

The formation of fraternal twins starts out with

two separate eggs as opposed to one egg.

FRATERNAL TWIN FORMATIONEgg

Sperm

IDENTICAL TWIN STAGES

Chorion: the outermost membrane surrounding an embryo.

Amnion: the innermost membrane that encloses the embryo.

The splitting occurs in days 1 through 3: the twins will each develop their own amnion, chorion, and placenta.

TWO-CELL STAGE

Amniotic Sac: the fluid-filled sac that contains and protects a fetus in the womb, made up of the amnion and chorion.

Placenta: a flattened circular organ in the uterus, nourishing and maintaining the fetus through the umbilical cord.

The splitting occurs in days 4 through 6: the twins will each develop their own amnion, but will share a chorion, and placenta.

EARLY BLASTOCYST STAGEThe splitting occurs in days 7 through 9: the twins will each share one amnion, chorion, and placenta.

LATE BLASTOCYST STAGE

Twins are rare and special, occurring in about 2% of all pregnancies. Of that number, 30% are identical twins; the other 70% are fraternal.

A common misconception about the conceiving of identical twins is that the trait for having them is passed on to future generations through the mother’s genes. But the truth is science doesn’t know the reason why identical twins occur. At this time, we can just say that they’re examples of a nine-month double miracle.

Identical twins have the same DNA, but di�erent fingerprints.

Twins have a higher probability of being left handed. 22% percent of twins are left handed, compared to 10% in singletons.

The state of Massachusetts has the highest rate of twins in America.

Women who eat more dairy, women who are tall, and women who are over the age of 30 have higher chances of having twins, and mothers of twins tend to live longer.

China has the lowest rate of twins averaging 1 in 300.

Twins can have di�erent dads, can be di�erent races, and even be born in di�erent years.

Male twins will not have twins unless their wives have the ability to produce two eggs.

Fraternal twin girls have twice the chance of giving birth to twins than singletons.

Every person on earth has a unique smell, except for identical twins.

If twin females and twin males marry, and they both have a child, both children will look the exact same.

The o�spring of two sets of identical twins are legally cousins butgenetically siblings.

Once you have fraternal twins, you are 3 to 4 times more likely to have another set!

Moms who are expecting twins tend to have higher levels of morning sickness, likely because of increased levels of the hormone human chorionic gonadotropin (HCG).

They tend to be early. Over 50% of twins are born before 37 weeks.

Did you know that Ashton Kutcher, Vin Diesel, Scarlett Johansson, and Alanis Morissette each have a fraternal twin brother? The average birth weight

for a twin is 5 lbs, 5 oz.

WOMB MATES