Having a multiple pregnancy carries with it a certain number of very specific risks. As a mom of multiples, you can expect to have more testing, more ultrasounds, and more monitoring than if you were carrying only one baby. Your pregnancy is, from the beginning, considered to be high risk. You are at a higher risk for developing iron-deficiency anemia, preeclampsia, gestational diabetes, and a whole host of other complications. In addition, there are specific concerns that a multiple pregnancy has in regard to miscarriage that make it more aggressive than with a singleton.
Statistically speaking, miscarriage will occur among multiples at around three times the rate that it does with singleton pregnancy. The rate of miscarriage is even higher if the multiples are from the same egg. In addition, it is possible for miscarriage to only affect one baby. The idea of a "vanishing twin" is thought to occur in around 1 in 5 twin pregnancies. In this sort of miscarriage, one twin is reabsorbed and the other is left to (hopefully) develop as normal.
A good example of how treatment for a multiple pregnancy is more aggressive than with a singleton is the example of an incompetent cervix. Having an incompetent cervix can complicate a single pregnancy, to be sure; however, because of the increased pressure and weight that multiple babies will place on the floor of the pelvis as the babies grow, the more dangerous an incompetent cervix becomes. Aggressively diagnosing and treating an incompetent cervix is especially important for a multiple pregnancy for this reason.
There are other reasons that miscarriage may be more aggressive with multiples than with a singleton. Many multiple pregnancies in the last few decades are due to a variety of fertility treatments. In many cases, these fertility treatments are undergone by women who are older than the average childbearing age. Having said that, older women are much more likely to experience a miscarriage than younger women, even if they have a singleton pregnancy. Naturally, then, you cannot underestimate the impact that fertility treatments have had on the miscarriage of multiples.