There is evidence from human studies that anxiety or stress during pregnancy can affect birth outcome, causing babies to be born earlier and possibly smaller for gestational age. There is also some suggestive evidence for longer-term behavioural problems. Animal studies indicate that antenatal stress does have a long-term effect on the behaviour of the offspring, including a hyper-responsive hypothalamo-pituitary -adrenal axis. The human foetus can mount an independent stress response from mid-gestation. Two possible mechanisms have been demonstrated by which maternal stress or anxiety may affect the human foetus, the passage of cortisol across the placenta, and an impairment of blood flow through the maternal uterine arteries.