The exact function of histamine during pregnancy is unclear, however, it is thought to play a role in embryo-uterine interactions due to its vasoactive, differentiation and growth-promoting properties.
The histamine-degrading enzyme diamine oxidase (DAO) is produced in high amounts by the placenta, though to prevent too many bioactive amines (like histamine) from entering the placenta. DAO is 500-1000 times higher during pregnancy than at other times.
If there is a barrier to the production of DAO in pregnancy, such as a genetic predisposition to low levels (deficiency), complications with the pregnancy can occur.
There appears to be a role for histamine in gestation, since there is a high expression of histamine-producing enzyme histidine decarboxylase in the placenta, histamine receptors at the feotal-maternal interface, and the existence of embryo-derived histamine-releasing factor (EHRF).
There appears to be a natural balance between histamine and DAO during an uncomplicated pregnancy.
Risks of reduced DAO activities include diabetes, miscarriage, threatened miscarriage, and trophoblastic disorders. One study looked into whether those with histamine intolerance experience more complicated pregnancies and higher pregnancy loss due to impairments to the DAO enzyme systems. Additionally, the researchers asked if low DAO levels or genetic modifications to the DAO gene might allow predictions for higher risk of miscarriage. The results showed individual results vary, thus limiting DAO activity as a risk screening tool.
There are known links between higher blood histamine levels and pregnancy complications such as preeclampsia, miscarriage, preterm labour and severe vomiting (hyperemesis gravidarum). Symptoms may appear the same as those non-pregnant individuals with experimentally induced high histamine.
It is normal for histamine levels to decrease in a normal pregnancy, likely because the placenta is supercharging histamine release, thus DAO is increased. Some people may find that allergic symptoms decrease during pregnancy because of this. If DAO enzyme action is impaired, histamine levels will increase beyond what we would see in a healthy non-pregnant individual.
Laura Maintz, Verena Schwarzer, Thomas Bieber, Katrin van der Ven, Natalija Novak, Effects of histamine and diamine oxidase activities on pregnancy: a critical review, Human Reproduction Update, Volume 14, Issue 5, September-October 2008, Pages 485–495, https://doi.org/10.1093/humupd/dmn014
Brew O, Sullivan MH. The links between maternal histamine levels and complications of human pregnancy. J Reprod Immunol. 2006 Dec;72(1-2):94-107. doi: 10.1016/j.jri.2006.04.002. Epub 2006 Jul 24. PMID: 16860879.