Does Your Immune System Weaken When Pregnant?

Pregnancy is often described as a time when the immune system is weakened, but immune experts say that picture is misleading.1PubMed Central. The Immune System in Pregnancy: A Unique Complexity The immune system is modulated rather than suppressed, so some defenses are turned down while others are turned up.2PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella The changes also follow a precise, predictable schedule across the nine months rather than a steady decline.3News Center. Immune system changes during pregnancy are precisely timed Whether you are more vulnerable depends on which kind of germ you meet and how far along you are.

What Actually Changes in a Pregnant Immune System

The white blood cells that make up much of your targeted immune defense keep a stable overall count in pregnancy.4PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella What changes is the balance. It tilts away from defenses that attack infected cells directly and toward antibodies, proteins that tag germs in the blood and body fluids.5PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella Immune cells in a pregnant woman’s blood make fewer of the chemical signals that drive cell-killing defenses and more of those that support antibody responses.6PubMed. Shifts in the TH1/TH2 balance during human pregnancy correlate with apoptotic changes That tilt is still present shortly after delivery, so it isn’t limited to the nine months themselves.7PubMed. Shifts in the TH1/TH2 balance during human pregnancy correlate with apoptotic changes Pregnancy quiets cell-killing defense and leans on antibodies, a trade that helps protect the baby, whose cells are partly foreign to your body.

This is why “weakened” and “changed” lead to different expectations. A weakened system would make every infection more likely. A re-tuned one raises risk only for certain germs. You still fight off most everyday colds about as you would otherwise, and you still make antibodies well, which is why vaccines are recommended. The real concern is a short list of germs that hide inside your cells.

Hormones drive the shift. Rising progesterone and estrogen push the immune system toward the antibody-favoring style.8PubMed. Th2 immune deviation induced by pregnancy: the two faces of autoimmune rheumatic diseases Progesterone has repeatedly been shown to dampen the cell-killing style of response.9PubMed Central. Impact of progesterone on the immune system in women: a systematic literature review Both hormones peak in the third trimester, so the shift is strongest late in pregnancy.10PubMed. Th2 immune deviation induced by pregnancy: the two faces of autoimmune rheumatic diseases Even in the normal menstrual cycle, the antibody-favoring signals rise from about 35% to 78% of the balance after ovulation, when progesterone is higher.11PubMed Central. Impact of progesterone on the immune system in women: a systematic literature review

Which Infections Become More Dangerous

Germs that hide inside cells take advantage of the quieter cell-killing defense. Listeria is a bacterium found in some foods that can live and grow inside human cells. The body controls it mainly with cell-killing defenses, so any condition that weakens them, including pregnancy, makes infection more likely.12PubMed Central. Listeriosis in Pregnancy: Diagnosis, Treatment, and Prevention That’s why pregnant women are commonly advised to avoid the foods most often linked to Listeria: unpasteurized milk and soft cheeses, deli meats unless heated until steaming, and refrigerated smoked fish. Listeria can cause only a mild flu-like illness in the mother yet still harm the pregnancy, so a fever after a risky food deserves a call to your doctor.

Listeria is unusual among food germs because it survives and multiplies at refrigerator temperatures, so keeping food cold doesn’t make a contaminated item safe. Heating food until steaming hot kills it, which is why the advice on deli meats is to heat them rather than skip them entirely. Symptoms can appear days to weeks after eating a contaminated food, so mention a risky meal to your doctor even if the fever starts much later.

Respiratory viruses hit harder. Viral lung infections such as influenza, COVID-19, and chickenpox are treated as higher-risk in pregnancy,13PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella though responses differ depending on both the germ and the stage of pregnancy.14PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella Pregnancy also changes breathing and heart workload, which can make lung infections harder to cope with. A cough with shortness of breath or a high fever is worth prompt medical attention rather than waiting it out.

Antiviral treatment for influenza works best when started soon after symptoms begin. A fever should be reported rather than quietly treated at home, since it can signal an infection that needs evaluation. Mild breathlessness from the growing uterus is common, but breathlessness that is sudden, worsening, or paired with fever or chest pain is not. And if you were exposed to chickenpox and aren’t sure of your immunity, contact your provider promptly, since a blood test can show whether you’re protected.

Wash your hands after handling raw meat, soil, or cat litter, and cook meat and eggs thoroughly. Stay away from anyone with chickenpox if you’ve never had it or the vaccine.

Feeling stuffy is often pregnancy hormones swelling nasal tissue rather than infection. Ask before taking any cold medicine, since some aren’t advised in pregnancy.

Why Your Body Doesn’t Reject the Baby

A transplanted organ from a genetically different person is normally attacked unless drugs suppress the immune system, yet a baby with half its genes from the father is not. Pregnancy solves this without drugs, using several layers of tolerance rather than one off switch. The baby isn’t fully hidden, either: maternal immune cells meet fetal cells at the womb lining, and tolerance is actively worked out there.

The paternal genes mean the baby’s cells carry markers the mother’s immune system could treat as foreign. So pregnancy produces calming immune cells that hold back immune attacks.15PubMed Central. Pregnancy induces a fetal antigen-specific maternal T regulatory cell response that contributes to tolerance These cells are specific to the baby’s markers and calm responses only when those markers are present.16PubMed Central. Pregnancy induces a fetal antigen-specific maternal T regulatory cell response that contributes to tolerance In mice, removing them led to fewer surviving offspring.17PubMed Central. Pregnancy induces a fetal antigen-specific maternal T regulatory cell response that contributes to tolerance Mice aren’t people, but human research points the same direction: such cells play a key role in enforcing tolerance between mother and baby.18PubMed Central. Maternal-Fetal Immune Tolerance, Block by Block

Immune cells in the womb also change their job. The womb lining fills with a type of immune cell that elsewhere destroys infected or abnormal cells.19PubMed Central. Uterine Natural Killer Cells: A Rising Star in Human Pregnancy Regulation In the pregnant womb they kill weakly despite carrying plenty of their weapons.20PubMed Central. Uterine Natural Killer Cells: A Rising Star in Human Pregnancy Regulation Instead, they release signals that help pregnancy succeed.21PubMed Central. Uterine Natural Killer Cells: A Rising Star in Human Pregnancy Regulation

The placenta adds its own protection. Its outer layer is a continuous sheet of fused cells with no gaps for many bacteria and parasites to squeeze through.22PubMed Central. TORCH infections at the maternal–fetal placental transmission: an overview of multi-omics, pathogenesis and innate immune defense It also constantly makes antiviral signals that help block viruses.23PubMed Central. TORCH infections at the maternal–fetal placental transmission: an overview of multi-omics, pathogenesis and innate immune defense Fetal cells can even stay in the mother’s body for years after pregnancy, which may help maintain tolerance.24PubMed Central. Forever Connected: The Lifelong Biological Consequences of Fetomaternal and Maternofetal Microchimerism

How Immunity Shifts From Trimester to Trimester

Early inflammation is needed because implantation involves the embryo burrowing into the womb lining, a process that resembles wound healing and needs inflammatory signals to remodel tissue. The second trimester suits growth, and many people feel their best then. It isn’t a period of complete immune shutdown, so the infection cautions still apply.

Not every tissue follows the same pattern. The placental area stays anti-inflammatory to protect the baby, but the gut lining shows rising low-grade inflammation as pregnancy advances.28PubMed Central. The Maternal Gut Microbiome during Pregnancy

Autoimmune Conditions During Pregnancy

Some conditions quiet down. Most women with rheumatoid arthritis improve or go into remission during pregnancy.29PubMed. Remission of rheumatoid arthritis during pregnancy and maternal-fetal class II alloantigen disparity Multiple sclerosis also frequently quiets.30PubMed. IL-12, TNF-, and Hormonal Changes during Late Pregnancy and Early Postpartum: Implications for Autoimmune Disease Activity during These Times Conditions driven by cell-killing immunity tend to ease when that side is turned down.31PubMed Central. Viral pulmonary infection in pregnancy – Including COVID-19, SARS, influenza A, and varicella Improvement isn’t guaranteed, though; some people still have symptoms or need medication throughout pregnancy.

Lupus often goes the other way. Most recent research finds it becomes more active in pregnancy, possibly due to hormonal shifts.32PubMed Central. Lupus Activity in Pregnancy More activity raises the risks of stillbirth, preterm birth, low birth weight, and preeclampsia.33PubMed Central. Lupus Activity in Pregnancy Many people with lupus still have healthy pregnancies, especially with planning and monitoring. People with lupus benefit from planning pregnancy with their specialist, ideally while the disease is quiet. Because preeclampsia is on the list, blood pressure checks and urine testing at visits matter.

For any of these conditions, talk to your specialist before conception about which medicines are safe. Don’t stop medication on your own, since a flare can be riskier than many drugs.

Vaccines in Pregnancy and Protection for Your Baby

Mothers’ antibodies are carried across the placenta to the baby during pregnancy.34PubMed. Human placental Fc receptors and the transmission of antibodies from mother to fetus A dedicated transporter in the placenta picks up one type of antibody from the mother’s blood and moves it into the baby’s blood,35PubMed Central. IgG Placental Transfer in Healthy and Pathological Pregnancies which is why this is the type passed on. Newborns are born with borrowed protection while their own immune systems mature. Transfer increases as pregnancy advances, so babies born very early receive less than those born at full term, and the protection fades over the first months of life.

COVID vaccination shows this in practice. Vaccine-made antibodies were found in all umbilical cord blood and breast milk samples in one study.36Harvard Gazette. Study shows COVID-19 vaccinated mothers pass antibodies to newborns Cord blood can hold more of these antibodies than the mother’s own blood, especially after first or second trimester vaccination.37PubMed Central. Maternal immune response and placental antibody transfer after COVID-19 vaccination across trimester and platforms Vaccination during pregnancy is linked to lower COVID hospitalization in infants under 6 months.38Morbidity and Mortality Weekly Report. Effectiveness of Maternal Vaccination with mRNA COVID-19 Vaccine During Pregnancy Against COVID-19–Associated Hospitalization in Infants Aged <6 Months — 17 States, July 2021–January 2022

This matters because newborns can’t be vaccinated against some diseases right away, so your antibodies cover the gap. Whooping cough is dangerous for very young babies, and passing antibodies against it is a main reason for vaccination in each pregnancy.

Breast milk carries antibodies too, but they work differently: they act mainly in the baby’s gut and throat rather than circulating in the blood. Finding vaccine-made antibodies in milk shows another route of protection after birth, which adds to the placental protection without replacing it.

On timing, flu vaccine can be given in any trimester during flu season. Whooping cough vaccine is usually given in each pregnancy around weeks 27 to 36. Live vaccines like chickenpox or MMR are avoided during pregnancy and given before or after. Ask your provider which vaccines fit your due date.

What Happens to Your Immune System After Birth

The calming effects lift abruptly. Late in pregnancy, key inflammatory signals run about 3 times lower than after birth for one and around 40% lower for another.39PubMed. IL-12, TNF-, and Hormonal Changes during Late Pregnancy and Early Postpartum: Implications for Autoimmune Disease Activity during These Times Those signals rebound after delivery as stress hormones and vitamin D levels fall.40PubMed. IL-12, TNF-, and Hormonal Changes during Late Pregnancy and Early Postpartum: Implications for Autoimmune Disease Activity during These Times As a result, rheumatoid arthritis and MS often flare, or even first appear, after birth.41PubMed. IL-12, TNF-, and Hormonal Changes during Late Pregnancy and Early Postpartum: Implications for Autoimmune Disease Activity during These Times Sudden immune and hormone changes plus medicine adjustments for breastfeeding can all trigger postpartum flares.42Frontiers in Lupus. Postpartum challenges in women with systemic autoimmune diseases Many people have no flare at all, so this is a reason for awareness rather than alarm.

Medicines add to the risk. Some people reduce or change them during pregnancy, and a returning strong immune system with less medication can raise flare risk. Breastfeeding choices affect which medicines can be taken, so decisions about both need to be made together. Resuming a suitable medicine soon after birth, as your specialist advises, can help prevent a flare.

To prepare, arrange a check-in with your specialist within the first months after birth, and learn beforehand which medicines are compatible with breastfeeding. New joint pain, rashes, or numbness deserve evaluation, not just a shrug about tiredness. Stress and an inflammatory diet may compound inflammation, so rest and good food help where possible.