Pregnancy: Immune System Abnormalities and Infections

For many years, scientists have thought of the mother’s immune system to be repressed during pregnancy to facilitate the growth of the foetus. Pregnant mothers tend to face altered or weakened immune systems and increased susceptibility to infections. 

Infection can be especially dangerous during pregnancy and can cause serious problems such as blindness, learning difficulties and heart complications in babies. 

Read on to find out how immune system abnormalities and infections affect mother and baby. 

What is the immune system doing in pregnancy? 

For the developing embryo to be implanted, foetal cells need to actively invade the lining of the mother’s womb. This causes a series of inflammation events, a process similar to when your body tries to heal from a wound. 

During the first 12 weeks of pregnancy, trophoblast cells actively recruit immune cells to the maternal-foetal interface to facilitate implantation and promote successful pregnancy. Trophoblast cells are the first cells to differentiate from a fertilised egg and provide nutrients to the embryo, developing into a large part of the placenta.

Immune cells are present at the maternal-foetal interface

In the mucosal lining of the endometrium of the uterus (maternal decidua) where the fertilised egg (blastocyst) implants, cells differentiate to accommodate implantation. In the next 15 weeks, some cells of the growing foetus express cell surface markers, or antigens, that originate from the father. Usually, the mother’s immune system will recognise and attack theses foreign cells. However, specialised white blood cells, regulatory T cells (Tregs), help to protect these foetal cells and suppresses the mother’s immune response to the foetus.  

Can immune system abnormalities explain unsuccessful pregnancy?

Antiphospholipid syndrome 

Antiphospholipid syndrome is an autoimmune disease disorder that affects pregnancy. This is characterised by antiphospholipid antibodies such as anti-cardiolipin antibodies, anti-beta-2-glycoprotein antibodies, and lupus anticoagulant. 

The presence of these antibodies causes clots (thrombosis) in veins, arteries and microvasculature. This can lead to pregnancy morbidity and placenta dysfunction which has implications on pre-eclampsia events, preterm birth and intrauterine growth restrictions. 

Miscarriage and low levels of regulatory T cells (Tregs)

In the final stages of pregnancy, the mother’s immune system heightens into a pro-inflammatory state to allow for labour to occur, similar to the first 12 weeks of pregnancy. 

In women who have recurrent miscarriages, Treg levels were also found to be significantly decreased. Low levels of Tregs could also contribute to preterm labour where there is dysregulation with the mother’s immune response. 

Haemolytic disease of the foetus and the newborn (HDFN)  

Haemolytic disease of the foetus and newborn (HDFN), or erythroblastosis foetal during pregnancy, is a blood disease in newborn babies where the infant’s red blood cells break down at a fast rate. It develops in a foetus or around birth when the mother and baby have incompatible blood types and Rh factors (where an Rh-negative mother has a baby with an Rh-positive father). 

The mother’s immune system perceives the foetus’ red blood cells as foreign and produces antibodies (IgG) against the foetus’ red blood cells. It usually happens when the placenta breaks away at birth or can happen anytime when blood cells are exchanged between the mother and foetus, for example, during a prenatal test when a needle is inserted or when the mother experiences impact from a fall. 

This causes red blood cells in foetal circulation to break down (haemolysis) and bilirubin levels to rise, causing jaundice (yellowing of the skin and eye whites) in the infant. Severity varies across different cases but can lead to serious complications such as heart failure and damage to the liver due to excess bilirubin. 

Luckily, HFDN can be prevented. Rh-negative mothers who have not been sensitised are prescribed Rh immunoglobulin (RhoGAM) that stops a mother’s antibodies from reacting with her baby’s Rh-positive cells. 

Preterm birth and Infection 

Infection can be associated with 50 to 80 per cent of preterm labour. Research suggests that infection leads to heightened levels of activation of the mother’s immune system, or increased levels of inflammatory markers in the foetus, which causes irreversible foetal damage and abnormalities in development. The increase in inflammation processes in the onset of infection can also lead to higher delivery rates within 48 hours. 

Harmful infections for mother and baby 

Infections such as syphilis, listeriosis, Group B streptococcus infection, hepatitis and HIV are especially harmful to both mother and foetus. 

Group B streptococcal infections occur in 25 per cent of women and can be transmitted to babies through vaginal deliveries. Therefore, doctors test women at the end of the pregnancy for this infection as it can cause stillbirth, serious infections and birth defects in the baby such as hearing and vision loss, sepsis, pneumonia and mental impairment. 

Hormones and infections 

Hormonal fluctuation in the mother also increases the risk for infection in the mother, especially in the urinary tract. Urinary tract infections are one of the most common infections that occur during pregnancy. 

During pregnancy, the uterus expands, and the body produces very high levels of progesterone, a hormone that helps the fertilised egg stay implanted in the uterus and prevent contractions. However, progesterone also encourages the bladder muscles to relax. This means that urine tends to be held for longer periods in the bladder, which increases the risk of developing a urinary tract infection. 

Oestrogen, another hormone that is produced rapidly in the first trimester to maintain pregnancy, prevent miscarriage and help stimulate foetal maturation also increases the risk of yeast infections in mothers. 


Pregnancy affects the body’s systems and can make mothers especially vulnerable to infections and complications. Pregnant mothers should schedule regular appointments with their obstetricians and seek medical attention when feeling sick.

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