Yes, pregnancy makes you more vulnerable to infections and can make common illnesses harder to shake. Your immune system undergoes deliberate changes to protect the developing fetus, and those same changes reduce your ability to fight off certain bacteria and viruses. The shift isn’t dramatic enough that you’ll catch every bug going around, but it’s significant enough that specific infections pose a real and measurable increased risk.
Why Your Immune System Changes During Pregnancy
Your body faces a biological paradox during pregnancy: the fetus carries genetic material from both parents, which means your immune system could theoretically recognize it as foreign and attack it. To prevent that, your immune system dials down certain defenses while ramping up others. This isn’t a simple weakening. It’s a recalibration.
The biggest shift involves two branches of your immune response. Your body moves away from cell-mediated immunity (the type that attacks infected cells directly) and leans toward antibody-based immunity (the type that works through circulating proteins in your blood). This shift begins early in pregnancy and becomes more pronounced as the weeks go on. By the third trimester, your cells produce less of the signaling molecules that drive aggressive immune responses, while anti-inflammatory signals increase.
Hormones drive much of this. Rising estrogen and progesterone cause the thymus, the organ that trains immune cells, to temporarily shrink. This partially explains why the number of key immune cells (both helper T-cells and killer T-cells) drops compared to pre-pregnancy levels. Progesterone also actively suppresses the inflammatory arm of the immune system and promotes the production of anti-inflammatory compounds. Meanwhile, a special class of regulatory immune cells increases during early pregnancy and peaks in the second trimester to keep inflammation around the uterus in check during implantation and growth.
The practical result: your body becomes less efficient at fighting off intracellular pathogens like viruses and certain bacteria, even though other parts of your immune system continue working normally.
Infections That Hit Harder During Pregnancy
The immune shift doesn’t affect every illness equally. Some infections are dramatically more dangerous during pregnancy, while others are only slightly worse than usual.
Listeriosis is the starkest example. Pregnant women are about 10 times more likely to contract listeria than other healthy adults, and roughly 1 in 6 of all listeria cases in the U.S. occurs in a pregnant person. The bacteria, found in deli meats, soft cheeses, and unpasteurized products, thrives precisely because the cell-mediated immune response that normally destroys it is suppressed. Hispanic pregnant women face an even higher risk, at about 24 times the rate of other healthy adults, likely due to dietary patterns involving fresh soft cheeses.
Influenza also poses a greater threat. Pregnant women who catch the flu are more likely to develop severe complications and require hospitalization compared to non-pregnant women of the same age. The combination of reduced immune function and the physical demands pregnancy places on the heart and lungs (your heart pumps significantly more blood, your lung capacity decreases as the uterus grows) makes respiratory infections particularly risky.
Urinary tract infections become more common as well, especially between weeks 6 and 24. As your uterus grows, its weight presses on the bladder and can block urine from draining fully. Stagnant urine creates an ideal breeding ground for bacteria, and your suppressed immune defenses are slower to clear the infection before it takes hold.
Common Colds Last Longer
Even everyday illnesses like the common cold tend to linger during pregnancy. Your body’s reduced inflammatory response, the very mechanism that typically produces cold symptoms while fighting off the virus, also means your immune system clears the infection more slowly. You may find that a cold that would normally resolve in a week stretches out several days longer. The severity can also feel worse because your treatment options are more limited; many over-the-counter cold medications aren’t recommended during pregnancy.
Why Fever During Early Pregnancy Matters
Getting sick during pregnancy isn’t just uncomfortable for you. Fever during the first trimester carries specific risks for fetal development. A large study spanning over a decade found that maternal fever from a cold or flu was significantly associated with several birth defects, including neural tube defects like spina bifida, cleft lip, and limb abnormalities. The increased risk ranged from 1.2 to 3.7 times the baseline depending on the specific defect.
Critically, the study found that having a cold or flu without fever showed no association with birth defects. The fever itself, not the underlying illness, appears to be the driving factor. This is one reason why managing fever promptly during early pregnancy is important, and why preventing infection in the first place carries real stakes.
Vaccines Recommended During Pregnancy
Because your immune system is less equipped to handle certain infections, vaccination becomes an important protective measure. Three vaccines are currently recommended during pregnancy.
- Flu vaccine: Recommended during any trimester throughout flu season. For those in the first or second trimester, September or October is the ideal window. Third-trimester vaccination as early as July or August can also help protect the baby during the first months of life, before they’re old enough to be vaccinated themselves.
- Tdap (whooping cough): Recommended during every pregnancy, regardless of whether you’ve had the vaccine before. The optimal window is between 27 and 36 weeks of gestation, which maximizes the antibodies that pass to the baby before birth.
- RSV vaccine: A single dose is recommended between 32 and 36 weeks of gestation, administered seasonally (September through January in most of the U.S.), to protect infants from respiratory syncytial virus during their first six months. Unlike the other two, this one is not repeated in subsequent pregnancies.
All three vaccines work double duty: they protect you during a period of heightened vulnerability, and they pass antibodies to the baby, providing protection during the earliest months of life when the infant’s own immune system is still developing.
Practical Ways to Lower Your Risk
The immune changes of pregnancy are normal and necessary, so you can’t prevent them. But you can reduce your exposure to the infections that take advantage of them.
For foodborne illness, avoid deli meats unless heated to steaming, skip soft cheeses made with unpasteurized milk, and be cautious with pre-made salads and smoked seafood. These are the primary vehicles for listeria. Thorough handwashing before eating and after handling raw meat covers a wide range of other food-related bacteria.
For respiratory infections, the basics apply with more urgency than usual: frequent handwashing, avoiding close contact with people who are visibly sick, and staying current on recommended vaccines. During cold and flu season, these measures aren’t just about comfort. They’re about avoiding fever during a period when fever carries developmental risks for the baby.
For urinary tract infections, staying well-hydrated and urinating frequently helps prevent the urine stagnation that allows bacteria to multiply. If you notice burning during urination or a frequent urge to go with little output, early treatment prevents the infection from reaching the kidneys, which is a more serious complication during pregnancy.