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Having more than one baby in a single pregnancy, such as twins, triplets, or more, is a risk factor for cerebral palsy. Children born in multiples are more likely to develop the condition than those born singly.
This risk applies both to natural multiple pregnancies and those that result from infertility treatments. Multiple pregnancies increase the risk for a number of complications, and some of these may, in turn, increase the risk of cerebral palsy. When one fetus dies during a multiples pregnancy, this further increases the risk.
Preventing multiple births is not usually possible, although the chance of having multiples increases with infertility treatments.
However, when it is discovered that a woman is carrying more than one developing fetus, she and her doctor can work together to ensure the pregnancy and the babies will be as healthy as possible. Careful medical monitoring is often more important during multiple pregnancies.
What the Research Says
Several studies have been conducted that show there is an increased risk of cerebral palsy in the children that are products of multiple births.
For example, a study published in 1996 looked at the registered births in two English counties between 1982 and 1989. They found that there were 2.3 incidents of cerebral palsy per 1,000 singleton births, 12.6 per 1,000 in twins, and 44.8 per 1,000 in triplets. 
Not only do multiple births increase the risk of cerebral palsy, but the more fetuses that develop per pregnancy, the greater the increased risk. This and other studies have also found that when one of the twins dies, the increase of the surviving twin having cerebral palsy is increased even more.
Another study during the 1980s looked at 155,000 birth records in Northern California. The results showed that the risk of cerebral palsy was 12 times greater in multiples. When one twin died, the risk was 108 times greater. The study also saw that low birth weight also increased the risk of cerebral palsy. 
Since multiples are smaller than singletons, this could contribute to the cerebral palsy risk.
The researchers hypothesized that this may be related to whether the twins are fraternal or identical. Identical twins share the same placenta and this may make the risk of cerebral palsy greater when there are complications, such as one twin dying in the womb.
Factors in Multiple Births That Make Cerebral Palsy More Likely
There is a clear and long-established connection, from these studies and much more, that multiples are at a significantly increased risk of developing cerebral palsy as compared to singletons.
There may be many factors at work that make the children of multiple pregnancies more vulnerable to the brain damage that causes the condition. Two of the most obvious are low birth weights and premature births.
Just comparing singletons and twins, ten percent of singleton births are premature, while 58 percent of twin births occur three weeks or more before the due date. Being born prematurely is a major and well-known risk factor for cerebral palsy.
A premature baby may not be fully developed, which can lead to breathing problems and inadequate oxygen to the brain. A premature baby may also be at a greater risk for infections and other complications that may contribute to cerebral palsy
Babies born of multiple pregnancies are also more likely to have low birth weights, another important risk factor for cerebral palsy.
One study found that when one twin weights 20 percent less than the other, both twins have a 40 percent higher risk of developing cerebral palsy. In general, a child that weighs five and a half pounds or less at the time of birth has an increased risk of cerebral palsy.  The reasons are similar for those seen with preterm babies.
The Role of Infertility Treatments
In 2015, a survey reported by the National Center for Health Statistics found that multiple births had reached an all-time high in the U.S. For 2014, 3.5 percent of all babies were born as multiples. This is still a small share of the overall births, but it is a significant increase. 
From 1940 to the mid-1980s the rate had been steady at about two percent. One reason for the increase may be that women are generally having children later, and the older age of the mother increases the chances of multiple births. Less than two percent of pregnancies in teen mothers are multiples, while multiples account for six percent of births among women in their 40s.
A likely bigger contribution to the increase in multiples is the rise in the use of fertility drugs and treatments. Hormone therapies, fertility drugs, and in vitro fertilization all increase the odds of pregnancies involving multiples.
Experts believe that these contribute to about two-thirds of the increase in multiple births. Of course, along with the increase in multiples has come increased risk for cerebral palsy.
Reducing Cerebral Palsy Risk
Cerebral palsy is just one possible complication that may result from multiple births. These babies are at a greater risk of dying, being disabled, and having health problems as they age. Many multiples develop normally and safely, but there are more risks associated with these pregnancies, and prevention, if possible is important.
Since the risk for cerebral palsy increases so much for triplets, and higher multiples, preventing these types of pregnancies can significantly reduce the risk of cerebral palsy.
Preventing multiple pregnancies is not straightforward. Reducing the risk of multiples can be achieved by women getting pregnant at a younger age and by not using infertility treatments. However, having children later is an important choice that many women are making now. Not using fertility treatments could mean that a woman may not get pregnant at all.
Researchers are working on strategies for reducing the risk of multiple pregnancies with infertility treatments. 
The strategies are more social than medical, for instance, changing how an in vitro fertilization cycle is defined in such a way that encourages women to have only one embryo implanted at a time. Whether changes like this can be implemented and whether they will reduce multiple births remains to be seen.
If you are going through a multiple pregnancy, your children may be at an increased risk of having cerebral palsy, but there are things you can do to reduce that risk. The most important thing to do is to work with your doctor to ensure your pregnancy, labor, and delivery go as smoothly as possible and that any complications are detected and managed right away.
If you have birth to multiples and one or more child has cerebral palsy, there may be a possibility that your doctor did not take all steps necessary to make sure you had a healthy and safe pregnancy. If you suspect this is the case, let a cerebral palsy lawyer guide you and help you collect evidence and make a case.
- Cerebral palsy in twins: a national study. (2002, September 1). ADC Fetal & Neonatal Edition. BMJ Journals.
Retrieved from: https://fn.bmj.com/content/87/2/F122
- Grether JK , et al. (n.d.). Twinning and cerebral palsy: experience in four northern California counties, births 1983 through 1985. - PubMed - NCBI. National Center for Biotechnology Information.
Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/8233749
- Boghossian NS , et al. (n.d.). Birth weight discordance in very low birth weight twins: mortality, morbidity, and neurodevelopment. - PubMed - NCBI. National Center for Biotechnology Information.
Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/31312035
- Multiple births in US at all-time high. (2015, December 11). Pew Research Center.
Retrieved from: https://www.pewresearch.org/fact-tank/2015/12/11/twins-triplets-and-more-more-u-s-births-are-multiples-than-ever-before/
- Responding to Infertility: Lessons From a Growing Body of Research and Suggested Guidelines for Practice. (n.d.). PubMed Central (PMC).
Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5798475/