Here are some stats and an article with stats referenced.
The odds of having twins or multiples with no fertility intervention is 1/250 or 0.4% chance.
The odds of having multiples with fertility intervention =1/4 or a 25% chance.
Fertility drugs increase the chances of multiples by 62.5% . (25%/0.4%)
This is not an uncommon side effect.
Here is the Article:
How common are twins and other multiple pregnancies?
More and more common over the past two decades, largely because more women are taking fertility drugs and turning to treatments such as in vitro fertilization (IVF) to help them conceive. Most fertility treatments increase a woman's chance of having fraternal, or dizygotic, twins (when two eggs are fertilized by two different sperm) or higher multiples (case in point: the 1997 Iowa septuplets).
And while about 95 percent of multiple pregnancies are still twins and the twins birth rate is going up, the rate at which women are having higher multiples has been increasing even faster. Between 1980 and 1999, the rate of twin births in the United States rose by 53 percent (a woman's likelihood of conceiving twins these days is about 1 in 33). During this same period, the rate of triplets and higher multiple births went up by 400 percent (it's about 1 in 555).
Meanwhile, the likelihood of having identical, or monozygotic, twins (when one fertilized egg divides in half) has remained steady at about 1 in 250. Identical twins happen by chance and are not thought to be affected by fertility treatments. Historically, about a third of all twins shared the same genes. As of 2001, only about 12 percent of twins born in the U.S. were identical, and that number is likely to shrink even more as the proportion of fraternal twins continues to increase.
I'm taking a fertility drug. Am I likely to have more than one baby?
While it's not a sure thing, prepare yourself for the possibility. Nowadays, most women who give birth to triplets or more have undergone some type of fertility treatment. Here's why:
Fertility drugs stimulate your ovaries, increasing the odds that you'll release several eggs at the same time. On average, 20 to 25 percent of women taking gonadotropin-releasing hormone (Pergonal) will become pregnant with twins or more, for example. Chances are about the same if you get pregnant through IVF, because several embryos will be inserted into your womb to increase the odds that you'll conceive. Other fertility techniques such as GIFT (gamete intrafallopian transfer) and ZIFT (zygote intrafallopian transfer) are also more likely to result in multiple-baby pregnancies. On its own, IUI (intrauterine insemination), where sperm are injected into a woman's uterus with a syringe, is the only fertility treatment that doesn't increase the chances of conceiving multiples. But most women who undergo IUI also take a fertility drug.
What other factors affect the chances that I'll have twins?
While identical twins happen by chance and occur at the same rate throughout the world, there are several factors that influence your chances of having fraternal twins:
+ Heredity: If twins run in your family, you're more likely to have a set yourself. Your partner's family history doesn't appear to affect your odds of having twins.
+ Age: The older you are, the higher your chances of having fraternal twins or higher multiples. Some studies indicate that this may be due to the fact that older women tend to produce higher levels of ovulation-stimulating hormones, but other studies do not support the connection.
+ Race: Twins are more common than average in African Americans and less common in Hispanics and Asians.
+ Nutrition: Being well nourished increases your chances of having twins and being mal-nourished decreases your chances.
+ Number of pregnancies: The more pregnancies you've had, the greater your chances of having twins. In fact, by your fourth or fifth pregnancy, your likelihood of having twins is four times higher than it was for your first pregnancy.
+ History of twins: Once you have a set of fraternal twins, you're twice as likely to have another set in future pregnancies.
That's what happened to Geri Martin Wilson. Less than three years after she gave birth to twins Natalie and Tyler (conceived with the help of the fertility drug Clomid), she got pregnant again, this time without fertility drugs. At her ultrasound, her doctor told her she was carrying another set of twins. She was shocked. "I just leaned on the ultrasound table and said, 'What are the odds of that happening?'" she recalls. It was another set of boy and girl twins, Joshua and Emma.