The hormone oxyocin is released by the brain during sex, breastfeeding and labour (!)and fosters feelings of love, warmth and trust.
According to new research, women with low levels of the hormone oxytocin during pregnancy are more likely to feel down after their baby is born. Women with higher levels find it easier to adapt to motherhood.
The finding raises the possibility that oxytocin could be measured in mothers-to-be – and a top-up pill given to those found lacking the hormone.
Post-natal depression affects up to 19 per cent of new mothers.
For the research at the Swiss University of Basel, 74 healthy pregnant women had their hormone levels measured in the last two to three months of pregnancy. They also answered questions designed to pick up symptoms of depression and were questioned again a fortnight after giving birth.
The analysis found a clear link between low oxytocin in pregnancy and symptoms of depression after giving birth. In addition, women who felt down when pregnant were more likely to struggle after the birth, the journal Neuropsychopharmacology reports.
Dr Gunther Meinlschmidt, of the University of Basel, said future studies should look at whether boosting oxytocin in pregnancy cuts the odds of post-natal depression. He added that early identification of women at risk 'could allow for early preventative interventions and minimise adverse effects for the well-being of mother and child'.
There could, however, be a rather inconvenient side-effect to the treatment, as oxytocin is used in hospitals to induce labour.
Dr Carmine Pariante, an expert in the psychiatry of pregnancy and motherhood from King's College London, said: 'This study shows for the first time that levels of oxytocin – the "bonding" hormone – are reduced in pregnancy in women who will later develop post-partum depression.'
She added: 'This study confirms the notion that depression in the perinatal period often starts in pregnancy, and has profound effects on the mother-child relationship.'
Post-natal depression is more common in women with a history of depression and those lacking support or whose babies need extra care.