A melon strolling on two tendrils. (3)
Put together the large curves of the melon and the slender curves of the tendrils, and you've got a pregnant woman. It's not the most flattering comparison, but for Plath's purposes, it's just right.
O red fruit, ivory, fine timbers! (4)
In a poem full of metaphors about pregnancy, this line is more than little disturbing. All of these things are highly valued: elephants are poached; forests are illegally logged for fine timbers; and red fruit is something we'd like to snatch off a tree and eat. This is how, our speaker seems to say, some men think about women. Thinking about a pregnant woman as something to be desired and valued like any object is not very warm and motherly—we think it's kind of creepy.
This loaf's big with its yeasty rising. (5)
Here Plath's playing on the typical association of women and the kitchen. That old stereotype of women baking delicious bread for their families is turned on its head—now it's the babies that she's providing.