About 6% of women with postpartum depression are resistant to treatment, according to international researchers who say some women are at a higher risk than others. Using population health data from Sweden, the researchers looked at the demographics and pregnancy characteristics of nearly 60,000 women diagnosed with postpartum depression for the first time, and looked at who went on to meet the criteria for treatment-resistant PPD over the course of a year. They say women with a lower educational level, lower household income, single mothers, mothers pre-existing mental or physical health conditions and those who smoked in early pregnancy were at higher risk of not responding to PPD treatment, as well as those who had a C-section or preterm birth. The researchers say these results could be used to improve risk assessment for new mothers.