Dr Brian Harris, senior lecturer in medical psychology at the University of Cardiff, has found that the women most likely to suffer had high levels of progesterone before delivery which fall steeply in the days afterwards.
Writing in tomorrow's edition of the British Medical Journal, Dr Harris says the possibility of using hormone supplements to treat women with postnatal depression now needs to be researched.
He describes maternity blues as tearfulness, irritability hypochondriasis, sleeplessness, impairment of concentration, and headache in the 10 days after delivery.
He began the study because progesterone is known for its anaesthetic action and because its levels rise to several 100 times higher than normal when the baby is due before falling back again.
He asked women to monitor their levels of progesterone and another hormone, cortisol, using a saliva test, before and after their babies were born.
Samples from 120 women who had normal babies after vaginal deliveries were analysed. Cortisol levels were not found to be significant.
Dr Harris found there was a link between maternity blues, high antenatal progesterone, low post-natal progesterone and levels of progesterone that fell steeply when the baby was born.Reuse content