... The placenta is crucial for fetal development and it usually protects the unborn baby from the stress hormone cortisol. However, when the mother is stressed, the placenta becomes less protective and the mother's cortisol may have an effect on the fetus.
... maternal stress and anxiety can alter the development of the baby's brain. This in turn can result in a greater risk of emotional problems such as anxiety or depression, behavioural problems such as Attention Deficit Hyperactivity Disorder, and being considerably slower at learning. Some studies have even suggested that it may increase the likelihood of later violent or criminal behaviour. Their findings have suggested that the effects of stress during pregnancy can last many years, including into adolescence.
... The researchers say that the stress hormone cortisol may be one way in which the fetus is affected by the mother's anxiety during pregnancy. Usually the placenta protects the unborn baby from the mother's cortisol, by producing an enzyme that breaks the hormone down. When the mother is very stressed, this enzyme works less well and lets her cortisol through the placenta. ... the Imperial researchers' latest study suggests that the higher the level of cortisol in the womb, the lower the toddler's cognitive development or "baby IQ" at 18 months.