It's a charming poem, and reminds one how good Plath was on motherhood--and how positive she was capable of being. I imagine the nine-line stanzas (as the nine lines of Metaphors) are a deliberate nod to the months of pregnancy.
Another neat syllabic poem (I like when, as here, syllabics use extreme enjambments, even, say, hyphenating words--lines pushing and breaking at the syllable-count barriers), "Probation," by Averill Curdy in Poetry some years back:
http://www.poetryfoundation.org/archive/poem.html?id=10
It's in a Moore-ish-like cunningly-rhymed complex stanza form--5,7,6,6,4