Several animal studies indicate that 2,3,7,8-TCDD (dioxin) affects the levels of reproductive hormones. In adult men that have been exposed to dioxins at work, levels of testosterone were reduced. The ability of dioxin to affect testosterone levels also has implications for the unborn male. When pregnant rats were given just one dose of dioxin, on a particular day of pregnancy, the male offspring had lowered levels of testosterone and a reduced sperm count. Their sexual behaviour was also demasculinised and feminised. Female offspring had abnormalities of their genitalia and vaginal canals.

One study of women who had accidentally been exposed to high levels of PCB and dioxins while pregnant gave birth to boys who had slightly shorter penises at age 11-14.

Following an accident at a chemical plant in Seveso, Italy in 1976, the local population was exposed to high levels of dioxin (TCDD). Research shows that exposure of both parents was associated with a significant increase in the proportion of girls born.

In the 1970s, residents of Times Beach in Missouri, USA, were accidentally exposed to high levels of dioxins because waste oil which was sprayed on roads for dust control was contaminated. Children born to women who were pregnant during the incident, were found to have changes in the number of some immune system cells at the ages of 9-14, indicating that effects on their immune systems were long-term.

A recent study in The Netherlands focused on healthy women and their babies from the general population. Some of the women had higher levels of PCBs and dioxins in their bodies and breastmilk than others. It was found that infants who were exposed to higher levels of these chemicals in the womb, and via breastfeeding, had changes in the number of certain immune system cells. It is not known what effect such changes could have on their health.

Inuit women inhabiting Arctic Quebec have relatively high body levels of persistent organochlorines because of their seafood-rich diet. Research has shown that higher levels of PCBs and dioxins in their breastmilk were linked with an increased incidence of ear infections in their babies. Other studies on women who ate fish from the Great Lakes in North America before becoming pregnant, reported that higher body levels of PCBs in the women were associated with an increased incidence of bacterial infections in their babies).

In humans, exposure to PCBs and dioxin in the womb has been associated with changes in thyroid hormone levels. In the Netherlands, a study was undertaken on healthy women and their babies from the general population who lived in Zandam region, near Amsterdam. The study calculated a baby's exposure to PCBs and dioxins in the womb by measuring the level of these chemicals in the mothers. It found that babies who had a higher exposure to PCBs and dioxins in the womb had slightly altered thyroid hormone levels.

Thyroid and sex hormones play a role in controlling development of the nervous system both in the womb and during childhood. A disturbance in the levels of these hormones at critical times when the nervous system is developing can cause changes which may, in extreme cases, even result in permanent brain damage. Possible effects of disruption of the thyroid hormones include low birth weight, hyperactivity, impaired learning and memory.