This is a very interesting study with some findings that can be a fantastic theme for a nice discussion.

When several countries of the developed world are faced with aging populations and where first-time mothers are also getting older and facing more pregnancy complications¹ it is interesting that this data shows that there may actually be some advantages to it. Although the authors perform only an observational study with a bit of trying to find a biological mechanism, and in no way are endorsing later-pregnancies as 'better', it might be good to bear in mind that the older a woman gets the higher are her chances of having complications during pregnancy.

But back to this study. The authors point-out that this is indeed a very particular group of women, and as such the findings must be restricted to them. Although their ethnic background is given as New Zealand European (and I'm guessing that the majority of the New Zealand Europeans are of Northern European descent), it would be intesting to know if there were any Southern Europeans in the mix. Also, I would just like to point out that although the mothers all belonged to the same socio-economic background, it is hard to believe that their home environments were similar. This is a wild guess, but I would imagine that younger mothers would probably have more trouble dealing with the stresses of a career in its beginning and a family life being initiated, with some of the younger ones, most likely, still studying. Women in their mid 30's to 40 will probably have their lives a little more settled. Also, these mothers probably have had more than one child and are better aware of how to take care of one.

So, although I think these results are interesting and certainly deserve to be discussed, my opinion is that there are a few factors that were not taken into account, and that are certainly influencing the outcomes.