Symbolism of becoming a chalice (metaphor): Connects to the previous image of Handmaid's needing to empty themselves of all to become a perfect vessel. Being nothing but a container, the chalice symbolises this as it is formally used religiously to hold sacramental/holy wine -- just as Offred is allowing herself to be emptied, mind and body, to carry something sacred (a child) and none more.

Also, because of the communion, it also means that she is turning herself over to God (drinking the blood).

Also, the shape of the chalice is like one of a woman's uterus.