Why does pregnancy change Mom's allergies?
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Can pregnancy really change a mother's allergies? Some mothers say: every spring they sneezed nonstop. Pregnant, it stopped. After the birth, it was back. The bigger worry: if Mom has allergies, will the baby too?
Let's follow a guard inside the body, through three stops. Meet the guard. It stands at the body's door and sounds the alarm when germs show up. Its name is the immune system. An allergy is the guard grabbing the wrong person. Pollen, cat hair, mango mean no harm, but it sounds the alarm anyway. Sneezing and rashes: that's the guard fighting innocent visitors.
First stop: pregnancy. Half of the baby comes from Dad. To the guard, that makes it half a stranger. By the old rules, it would treat the baby as an outsider. So in pregnancy, hormones give the guard new orders: turn some alarms down. But not all at once: it turns the knobs one by one. Some go quieter, some get louder.
Allergies and asthma work the same way. One study followed more than eight thousand pregnant women with asthma: about a third got better, a third got worse, and a third stayed the same. One common mix-up: many pregnant women have a stuffy nose. It's usually not a new allergy; hormones swell the blood vessels in the nose. It usually clears a few weeks after birth. Second stop: after the birth. Within days, the hormones drop away and the knobs swing back.
In one disease, the guard attacks the body itself. A study of over two hundred pregnant patients found: in the last three months of pregnancy, flare-ups fell by seventy percent; in the first three months after birth, they were more common than before pregnancy. Allergies often do this too, coming back after birth. As for brand-new allergies after birth: there's little research. No one can say whether pregnancy is behind them. Third stop: the baby. If Mom is allergic to mango, will the baby be allergic to mango too?
Studies of many twins found: genes mostly decide how jumpy the guard is, not which thing it hates. So Mom reacts to mango. The baby might react to eggs, get eczema, or react to nothing at all. A common estimate: neither parent allergic, a bit over one in ten kids. One parent, two to four in ten. Both parents, four to six in ten. The numbers vary, but they point the same way.
And the American Academy of Pediatrics notes: avoiding foods during pregnancy doesn't prevent allergies in the child. So what gets passed down isn't the mango. It's a slightly jumpy guard. Did you used to think a child inherits Mom's exact allergy? Tell us in the comments.
Gone while pregnant, back after birth
Some moms say: every spring I used to sneeze, then I got pregnant and it went away, and after the birth it came back. Others find the opposite — pregnancy made it worse.
And the bigger worry: if Mom is allergic, will the baby be too?
Let's follow a guard inside the body through three stops: pregnancy, after birth, and the baby.
The guard who thinks a mango is a bad guy
A guard stands at the body's door. When it sees a germ, it sounds the alarm. Its name is the immune system.
An allergy is the guard grabbing the wrong person. Pollen, cat hair and mango mean no harm, but it sounds the alarm anyway. Sneezing and rashes are the guard beating up innocent guests.
Half the baby comes from Dad
Half of the baby comes from Dad. To the guard, that makes the baby half a stranger. By the old rules, it would chase the baby out.
So during pregnancy, hormones give the guard new orders: turn some alarms down.
But it doesn't turn everything down. It twists knobs one by one: some go down, some go up.
8,000 moms, split into three
Allergies and asthma work the same way. A US study followed more than 8,000 pregnant women with asthma: about a third got better, a third got worse, and a third stayed the same.
Doctors can't yet predict who will get better and who will get worse.
A stuffy pregnant nose usually isn't a new allergy
Many pregnant women have a nose that stays blocked. Usually it isn't a new allergy — hormones make the blood vessels in the nose swell. It typically clears up a few weeks after birth.
After birth, the knobs snap back
Within days of giving birth, the hormones drop, and the knobs get twisted back.
The clearest example is multiple sclerosis, a disease where the guard attacks the body's own nerves. In a study of more than 200 pregnant patients, flare-ups fell by 70% in the last three months of pregnancy, then in the first three months after birth were even higher than before.
Allergies often do this too: they come back after birth.
Brand-new allergies after birth are barely studied, so no one can say if pregnancy is to blame.
Mom reacts to mango; the baby may not
Studies comparing many pairs of twins found that genes mostly set how jumpy the guard is, not which thing it hates.
What gets passed down isn't the mango. It's the jumpy guard.
How many allergic parents, how much risk
These are common estimates from allergy clinics. The numbers vary from source to source, but the direction is the same: the more allergic parents, the jumpier the child's guard tends to be.
Jumpy doesn't mean certain. Even with no allergic parents, more than 1 in 10 children become allergic.
Skipping foods in pregnancy won't block allergies
The American Academy of Pediatrics said in 2019: avoiding foods during pregnancy or breastfeeding does not prevent allergies in children. Eat normally.
More on the pregnant body: why pregnancy makes you nauseous and how the body knows it's time to give birth.
Quick check
Mom is allergic to mango. What is the baby most likely to inherit from her?
Genes mostly set how jumpy the guard is, not which thing it hates — so the baby might react to eggs, get eczema, or react to nothing at all.
