Why a pregnant woman with a dizygotic twins is said to be more fertile than of the monozygotic twins?

I had never heard that say before, but it makes sense to me.

Having dizygotic twins is having 2 different kids at the same time (two distinct ovums fertilized by 2 distinct spermatozoids at the same time). The twins are as different as 2 non-twin siblings can be, and as alike too.

Monozygotic twins are coming from 1 ovum fertilized by 1 single spermatozoid. The fertilized ovum will become two a little later. It is the same as giving birth to 1 kid, but the kid is doubled ... But that's an odd way to present things.

It's like in the first case, the twins would could have been borned at a different time, so a calculated index based on the number of kids by a range of time would increase with dizygotic twins, while the monozygotic twins are meant to be born at the same time and should not be considered as 2 for such a formula.

But, is that so important at the end ?

Studies have shown that women who are over 35 years have higher chances of getting pregnant with non-identical twins. This is likely because woman's ovaries start releasing more than one egg each month,. Another cause is that older women had higher levels of a hormone called FSH (follicle-stimulating hormone)

Its not about being more fertile, there are many other factors.

Factors that influence your chances of getting pregnant with twins are

What’s causing this multiple-baby boom? Experts say the following are a few of the factors behind twin birth:

Family history

If twins run in your family, then you have greater chances of carrying twins. Heredity doubles your chances if it’s on your mom’s side. Your partner’s family tree might be bursting with twins but this won’t increase your odds. 

Ethnicity

Ethnicity is another factor that can increase your chances of carrying two babies aboard your mothership. Black women are more likely than Caucasian women to give birth to twins. Asian and Hispanic women have even fewer chances than Caucasians to get pregnant with two or more kids at a time. 

The highest rates of twin births are seen in Central Africa, especially in the Yoruba ethnic group, with more than 18 twins per 1,000 births. The twinning rates in Asia and Latin America are less than eight per 1,000 births.

Several pregnancies

Your odds of having twins go up with each pregnancy.. And if you've had given birth to twins before, you're more likely to do so again.

Age

Being an older women can help. Studies have shown that women who are over 35 years have higher chances of getting pregnant with non-identical twins. This is likely because woman’s ovaries start releasing more than one egg each month,. Another cause is that older women had higher levels of a hormone called FSH (follicle-stimulating hormone).

Weight

Some researchers claim that women with a higher body mass index (BMI) have higher chances of carrying twins. A BMI of 25 or higher is termed overweight, while 30 and above is considered obese. Women who were obese before pregnancy are significantly more likely to give birth to non-identical twins, However, doctors don’t advise women who want twins to gain weight.