Pregnancy does make you more vulnerable to certain infections, but not in the way most people think. Your immune system isn’t weakened during pregnancy. It’s reorganized, shifting how it responds to different threats so your body can tolerate a growing baby that carries foreign genetic material. That reorganization leaves some gaps, making specific illnesses hit harder or occur more frequently while leaving other defenses intact or even enhanced.
Your Immune System Shifts, Not Shuts Down
For decades, even medical textbooks described pregnancy as a state of immune suppression. That framing is outdated. Researchers now describe it as immune modulation: your body dials down certain responses and ramps up others. The distinction matters because calling pregnancy an immunocompromised state overstates the risk for most infections and can lead to unnecessary anxiety or, worse, inappropriate medical decisions during outbreaks.
What actually happens is driven by hormones. Progesterone, estrogen, and a hormone called hCG (the one pregnancy tests detect) all bind directly to immune cells and change their behavior. Progesterone reduces the ability of certain white blood cells to produce powerful inflammatory molecules. It slows the migration of immune cells into the uterus and even triggers some of them to self-destruct. Estrogen reshapes the balance of immune cell populations throughout the body. Together, these hormones suppress the branch of immunity that would normally attack foreign tissue, which is exactly what the placenta and fetus look like to your immune system.
This tradeoff is precise. Your body reduces its antigen-presenting capacity (the ability of immune cells to flag invaders for destruction), dials back natural killer cell activity, and boosts populations of regulatory cells that actively promote tolerance. At the same time, other parts of your immune system stay fully functional. Neutrophils, the first-responder white blood cells, actually increase steadily from the first trimester onward. Complement proteins, which help kill bacteria directly, rise in the second and third trimesters. So the picture isn’t one of blanket vulnerability. It’s selective.
Where the Real Risks Are
The infections that pose genuine increased risk during pregnancy fall into a few categories: foodborne illness, urinary tract infections, and certain respiratory viruses. Each one exploits a different aspect of the immune shift.
Listeriosis is the standout foodborne risk. Pregnant women are about 10 times more likely to develop a listeria infection than other healthy adults, and pregnant Hispanic women face a 24-fold increase. Roughly 1 in 6 of all listeria cases in the U.S. occurs in a pregnant person. The bacteria can cross the placenta, and the consequences for the fetus are severe: stillbirth, preterm delivery, and life-threatening neonatal infection. This elevated risk is why food safety rules during pregnancy are so specific about deli meats, soft cheeses, and unpasteurized dairy.
Urinary tract infections become more common for purely mechanical reasons on top of the immune changes. Progesterone relaxes the smooth muscle lining your ureters and bladder, which slows urine flow. As the uterus grows, it physically compresses the bladder, reducing its capacity and sometimes partially blocking the ureters. Urine that sits in the urinary tract longer gives bacteria more time to multiply. Hormonal changes also alter vaginal flora in ways that increase colonization of the bacteria that cause UTIs. The combination of stagnant urine, physical compression, and a slightly dampened cell-mediated immune response makes UTIs both more likely and more dangerous during pregnancy, since they can progress to kidney infections more easily.
Flu and COVID-19: More Likely to Be Hospitalized
Influenza hits pregnant women harder than their non-pregnant peers. A large meta-analysis of observational studies found that pregnant women with the flu were about 2.4 times more likely to be hospitalized. Interestingly, the same analysis found no significant increase in mortality, ICU admission, pneumonia, or need for mechanical ventilation. So the flu is more likely to land you in a hospital bed during pregnancy, but it doesn’t appear to carry a higher risk of the worst outcomes.
COVID-19 tells a similar but more nuanced story. During the Delta wave, symptomatic pregnant women had roughly 2.4 times the risk of death compared to symptomatic non-pregnant women of reproductive age. That risk has decreased substantially with less virulent strains and broader population immunity from both vaccination and prior infection. Pregnant people with pre-existing conditions like diabetes, high blood pressure, or obesity face even higher risk from COVID, consistent with the general population. COVID during pregnancy is also linked to increased rates of preterm birth. The reassuring finding is that babies born to mothers with COVID don’t appear to have higher rates of being small for gestational age, stillbirth, or birth defects compared to unexposed pregnancies.
How Infections Can Affect the Baby
The stakes of getting sick during pregnancy extend beyond how you feel. Infections can affect the pregnancy itself and the baby’s health in ways that range from mild to devastating, depending on the type of infection, its severity, and when during pregnancy it occurs.
Preterm birth is the most common serious consequence. Vaginal, cervical, and uterine infections are associated with the majority of very early preterm births (before 28 weeks). In developed countries, 10% to 25% of all stillbirths appear to be caused by infection, and the earlier the loss occurs, the more likely infection played a role. One study found that 19% of fetal deaths before 28 weeks were infection-related, compared to just 2% of term stillbirths.
The downstream effects of infection-related preterm birth can be significant. Intrauterine infection has been associated with a doubling of the risk for brain bleeding and other neurological complications in preterm infants. Infection of the membranes surrounding the baby has been linked to a tripling of cerebral palsy risk in low-birth-weight infants. These are not everyday risks for a typical healthy pregnancy, but they underscore why preventing and promptly treating infections during pregnancy gets so much clinical attention.
How Your Risk Changes by Trimester
Your immune profile isn’t static across all nine months. The first trimester features higher levels of certain suppressor T cells and reduced expression of genes that help immune cells present foreign material for attack. Monocytes in the first trimester show increased activity of anti-inflammatory pathways and reduced production of infection-fighting signals.
By the third trimester, the changes look different. The body shifts toward more classical monocytes (the type associated with initial immune responses) and away from the patrolling non-classical type. When stimulated, third-trimester monocytes produce less of the inflammatory signals that drive fever and pathogen killing. Helper T cells that produce a key antiviral molecule decline in the third trimester, while a sharp 60% drop in certain inflammatory T cells occurs between the first and second trimesters.
In practical terms, this means the third trimester tends to carry the highest risk for complications from respiratory infections like flu and COVID. It’s why vaccination timing is planned to build protection before that vulnerable window.
Practical Steps That Reduce Your Risk
Most infection prevention during pregnancy comes down to food safety, hygiene habits, and vaccination.
- Cook meat thoroughly. Ground meat to at least 160°F, poultry to 165°F, and other meats like beef and pork to 145°F with a rest period. No pink, clear juices only. Don’t eat hot dogs or deli meats unless reheated until steaming.
- Skip unpasteurized dairy. Avoid raw milk, soft cheeses like feta, brie, and queso fresco unless the label confirms pasteurized milk.
- Wash your hands frequently, especially after handling raw food, using the bathroom, or being around young children (who are efficient germ carriers).
- Avoid cat litter and contaminated soil. Toxoplasmosis spreads through cat feces and soil. Have someone else handle litter duty, or wear gloves and wash thoroughly if you must do it yourself.
- Stay away from rodents, lizards, and turtles, including pet varieties. These animals can carry bacteria that pose particular risk during pregnancy.
- Get vaccinated. The flu vaccine is recommended for all pregnant women, with current guidelines specifying thimerosal-free formulations. Tdap vaccination during pregnancy protects newborns from whooping cough in their first vulnerable weeks of life.
The overall picture is more reassuring than alarming. Pregnancy doesn’t leave you defenseless. It recalibrates your immune system in ways that create specific, identifiable vulnerabilities. Knowing what those vulnerabilities are lets you take targeted precautions rather than spending nine months worried about every sneeze.