You could say the instinct to protect a baby starts when it’s in the womb.

Starting in the second trimester of pregnancy, a pregnant woman passes important disease-fighting molecules called immunoglobulin G, or IgG, through the placenta to her fetus. As the pregnancy continues, this sharing of immune protection gets further ramped up. The “stockpile” of protection is critical for helping newborns fight off potential infections from their first few hours of life through to about six months of age, when they’re most vulnerable to infections. While the newborn’s own immune system matures, and before the baby is old enough to be immunized against routine childhood diseases, maternal antibodies protect it, shown below. 

This maternal-to-fetus antibody transfer is among the most fascinating processes a woman’s body carries out to help her baby thrive after birth. “It's really an active process,” says Phil Dormitzer, Vice President and Chief Scientific Officer in Pfizer’s Vaccines Research & Development Unit in Pearl River, New York. “The placenta pumps the antibodies from a pregnant woman into the fetus, so you can end up with an even higher concentration of immune protection in the baby than is in the mother.”

Read on to learn how a pregnant woman passes life-saving immunity to her developing fetus.  

As shown in this graphic, to pass immune protection on to her baby, a pregnant woman needs to have acquired disease-specific antibodies by past exposure to a pathogen such as a virus or bacterium, or by vaccination. And sometimes her level of a specific antibody has diminished over time, depending on when she received a vaccine or last encountered the pathogen. That’s why the CDC recommends that pregnant women receive a booster Tdap (tetanus-diphtheria-acellular pertussis) vaccine with each pregnancy — to be able to pass sufficient antibodies on to their babies. They also recommend a flu vaccine during each pregnancy.

Maternal vaccination has become a powerful tool to help fight some of the major causes of infant mortality around the globe, and a number of diseases, such as neonatal tetanus, are now routinely prevented. None of the vaccines that are currently recommended for maternal immunization were developed to protect pregnant women; rather, the goal is to protect the baby. As scientists continue to increase their understanding of the diseases that affect very young infants, clinical trials have recently begun to study maternal vaccines to protect against respiratory syncytial virus, a cause of deadly respiratory infections in newborns, and group B strep, a bacterial infection that can lead to pneumonia, meningitis, and blood infections.