It says the study was published in the January 2010 edition of Acta Obstetricia et Gynecologica, a Scandinavian journal, and was a metastudy of the benefits of breastfeeding involving tens of thousands of women overall.
Except that it wasn't. There is one article by Sven Carlsen, but it's about hormone levels in breastfeeding women, involving a study of 181 women. It's not surprising he's interested in this, because he's at the Department of Endocrinology of Trondheim University Hospital.
The source of the PA article appears to be at news-medical.net, which makes it clear that the basic story is the one on hormone levels, but tags on Carlsen's comments about the benefits of breastfeeding at the end.
And it's that which has been picked up. As I am not about to pay to read the journal version, I don't know what studies the comments were about, but they were clearly not the focus of the article - this aspect is not even mentioned in the abstract!