Researchers analysed blood samples from 51 women, all with a history of mood disorders, who were followed during and after pregnancy. From a sub-group of 32 women without symptoms of depression during their third trimester, 11 became depressed within four weeks of giving birth. Twelve women from a sub-group of 19 who were depressed during pregnancy remained so after birth. Chemical alterations, known as 'epigenetic changes', to two genes were closely associated with the development of post-natal depression.
'We were pretty surprised by how well the genes were correlated with post-natal depression', said Dr Zachary Kaminsky, assistant professor of psychiatry and behavioural sciences at Johns Hopkins University School of Medicine, USA, and senior author of the study.
Levels of the hormone oestrogen drop following childbirth. Dr Kaminsky's team suspected that this could prompt epigenetic changes to genes that are important in the hippocampus, a part of the brain that helps regulate mood. These epigenetic changes alter the way genes function without interfering with the genetic sequence.
The researchers began by studying one epigenetic mechanism, DNA methylation, in the hippocampus of mice. Statistical models identified two genes – HP1BP3 and TTC9B – that looked likely to be involved in post-natal depression.
The clinical study gave early confirmation of this. Women who developed post-natal depression showed higher levels of DNA methylation to the two identified genes. However, one potentially confounding variable in the study was that all the women had histories of depression or bipolar disorder.
Next, the researchers hope to follow a larger sample of women for longer. Ultimately the team hopes to develop a blood test predicting a woman's risk of post-natal depression.
'If you knew you were likely to develop postpartum depression, your decisions about managing your care could be made more clearly', Dr Kaminsky commented.
In the UK, approximately one woman in seven develops depression after having a baby. NHS Choices notes that the persistent low mood 'usually develops in the first four to six weeks after childbirth, although in some cases it may not develop for several months'.