Efface

"I’m no more your mother
Than the cloud that distills a mirror to reflect its own slow
Effacement at the wind’s hand."
-Morning Song, Silvia Plath 

Not going to lie, I googled the definition of effacement. 

Effacement (verb)
1. to wipe out; to do away with; expunge

Here's a not so secret secret: Babies make you feel old. Nothing about having to care for every aspect of a childs life make you feel young. Nobody has ever gone "Wow, I feel so youthful and full of life when I have to get up a two a.m.!" I think what's happening is the baby is reminding Plath of her own mortality, and she's trying to distract herself from it. Many times people have children so they can live on in their children. You put so much of your time into your children, you've given so much of your self to them that it's hard for you not to live through them once you are dead. It's feels like Plath is saying "I'm not really your mother, I'm just raising you so I can forget my own eventual demise." Apparently it didn't work, or her head probably wouldn't have gotten so close to that oven. Too soon?