In this photo the blending between background and foreground is very soft, do you think that this is part of the setup or was done in post? If you think it's done in the setup, please explain how.
A better resolution image would help giving a definitive answer.
That said, I'm not sure that a long focal was used, I would think maybe 50mm. Regarding the depth of field, the foremost part of the "blanket" (very bottom of photo) is also sharp even though it is probably 15cm or more before the baby's face, whereas the "hat" gets blurred only a couple of centimeters further. It is possible with careful manual focussing (a few centimeters in front of the face), or a tilt-shift lens could have been used (allowing a focal plane not parallel to that of the film/sensor...)... but in fact the focal plane doesn't even seem a plane at all. The sharpness also seems to subside in a way that seems unnatural for purely lens shallow DOF, it's too "brutal" even by digital standards.
Additionnally, the blanket becomes strangely blurred and blended into the background just in front/right of the baby's hands, and below/left of the feet. This looks like post-production blurring to me, not purely optical.
Finally, there is no sharp interface between the baby and the background itself, which makes me suspect that the original background was completely replaced altogether with some masking/paintbrushing/...
All in all an interesting picture anyway (although a bit artificial looking ?).
It looks to me like they may have used a shaped filter over the lens to cause blurring near the edges of the image. It doesn't look like it could just be shallow depth of field because of how clear the blanket is in front of the baby. A shaped piece of semi-transparent material in front of the lens however could produce a blurring like this or it is possible the entire blurring was done in post.
It isn't a technique I personally am all that familiar with, however this blog post I found has a lot of great detail. Apparently it can also be accomplished by spreading vasoline or something similar on a UV filter. You either cut the filter or spread the oil in the shape you want the diffusion to be.
I'd say this is made with an Aperture of about ƒ1.8 or more likely ƒ1.4 . This way, you only have very little depth of field (the area what is sharp in the picture) and the back- but also the foreground (depending on your focal length and your distance to the subject) will be very soft.
Without any EXIF-Data contained within the Picture, you can't say for sure. But the Background Blur looks like a very high speed lens used wide open, e. g. a Canon 85mm f/1.2 or 135mm f/2.0 on a Full Frame Camera.