The only article I am aware of on the subject is:

Unfortunately I don't have a pdf of it and my hard copy is out of reach and it will be for 2-3 months more. If I remember well, this article gives you some formulae to calculate 'days after conception' based on the greatest length of some of the long bones. The downsides are the use of modern Norwegian Landrace pigs as a sample and the need to have whole and relatively undamaged foetal/neonatal specimens.

All the best,

Angelos Hadjikoumis