The diamine oxidase (DAO) enzyme is produced by the placenta in large quantities and acts as a metabolic barrier to prevent excessive bioactive histamine from the placenta entering the circulation of the mother or the fetus.

Therefore, it has been suggested that assessing the functionality of the DAO enzyme may be useful as a tool to detect the risk of pregnancy complications.

The endometrium, the lining of the uterus, and the myometrium, the outer muscular layer of the uterus, contain large numbers of mast cells that contain histamine (Massey et al., 1991; Pap, 2004), and mast cells are also present in the placenta (Purcell and Hanahoe, 1991).

Histamine contributes to embryonic-uterine interactions during implantation.

In the placenta, the expression of the histidine decarboxylase (HDC) enzyme is about 1000 times higher than in other organs (Pap, 2004).

The histidine decarboxylase gene is regulated by progesterone (Paria et al., 1998), which increases during pregnancy.

In humans, H1R receptors have been observed to be expressed in placental villi and play important roles in the exchange of substances between maternal and fetal blood and in the secretion of many hormones such as estrogens (Matsuyama et al., 2004).

Histamine plays a significant role in the fetal-maternal interaction after embryo implantation (Brew and Sullivan, 2001; Pap, 2004, Noskova et al., 2006).

In normal pregnancies, a significant increase in DAO levels has been described during the first trimester (Southren et al., 1964; Dubois et al., 1977), followed by a plateau during the second and third trimesters (Southren et al., 1964; Dubois et al., 1977).