That could be it, but I think another way of looking at it gets rid of the trouble of the word "mischief". I'll outline it, and see whether you think it's even remotely logical.
I looked up the bit about the "fruit depart" in an online concordance, and it was pretty clear that it means the child leaving the womb. Not necessarily death, simply the (premature) birth of the child. Given that, I think it would make some sense for there to be a fine, as a birth of that sort would be done without benefit of a mid-wife, adding another element of risk to an already dangerous proposition. Then, the whole "mischief" bit could be injury to the child that resulted from being born too early. If the child survives with no damage, then there would be no reason to punish the offender any further. However, if the child is maimed/killed/disabled in some way, then the offender was to receive the same injury that he inflicted.
Anyway, let me know what you think. I don't have access to the proper resources to look this up, but I will if I get the chance.