Apron is a term used for the lower belly - there is an operation called an apronectomy done to remove excess skin from there and restore a more youthful apearance.

As she is pregnant then perhaps the reference is not to a garment but her body?

In Willie of Winsbury shouldn't there be a line about the darkest place on his body being fairer than the lightest on mine, said by the lady's father?

Anne Croucher