Health authorities often warn pregnant women against eating certain foods. This is because some foods may contain microbes, or substances that can be potentially dangerous to the unborn child or to the mother. Cheese is one group of foods that a pregnant woman has to be wary of, though many cheeses are regarded as safe to eat. As of 2011, mozzarella in pregnancy may be safe, though it depends on whether the milk the cheese was made from was pasteurized.
Pasteurization is a heat treatment process that kills the majority of microbes in milk. It is the presence of certain types of microbes in cheese that poses a danger to pregnant women and their babies. The risk of a dangerous microbe infecting a woman is very low if the milk is pasteurized, and unacceptably high if the milk is unpasteurized. Mozzarella in pregnancy, therefore, should not be eaten if the product is made from unpasteurized milk.
Pasteurized mozzarella is considered safe to eat. Pregnant women can check this on the label of mozzarella products when buying it in a store. In a restaurant setting, unless the chef can provide assurances that the mozzarella is pasteurized, then it is unsafe to eat.
Hard cheeses, like cheddar, have low levels of water, and are not hospitable environments for many dangerous microbes, especially when they are made from pasteurized milk. Softer cheeses contain more water, and microbes tend to grow in these more quickly. Examples include blue cheese, feta, and Mexican queso. The most significant dangerous microbe that can be present in cheese, especially unpasteurized cheese, is Listeria monocytogenes.
Listeria monocytogenes is a bacterial species, and while it can grow in cheese, it can also be present in other foods, like delicatessen meats or raw milk. Normally, women who are not pregnant do not suffer severe foodborne illness by ingesting the bacteria. In pregnancy, however, due to the presence of a fetus that has different chemistry to its mother, a woman has a less robust immune system. If a woman ingests Listeria monocytogenes when she is pregnant, she can suffer significant health problems and even lose her baby.
Research is ongoing into food safety in pregnancy, and women who want to check that their diet poses no significant risk to their baby should always check the most up to date information from their national health authority. Although pasteurized mozzarella in pregnancy is considered safe to consume, a pregnant woman should take care to check other ingredients in a dish with mozzarella, in case these ingredients, like ham, may also be potential sources of Listeria monocytogenes. Other pasteurized soft cheeses that are as safe as mozzarella in pregnancy include cream cheese, ricotta and cheese spreads. Some health authorities also approve pasteurized blue cheeses, pasteurized goat's cheese and pasteurized brie, but others advise against these, even when pasteurized.