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.
Money's new-minted in this fat purse. (6)
Again, we see pregnant women as commodities, items of value. But the comparison gets more awkward and unfortunate when you remember that "purse" can be used as slang for a vagina, especially for a prostitute's vagina, because that's how they make their money, or fill their purse.
I'm a means, a stage, a cow in calf. (7)
Now the speaker is coming straight out and saying that she feels used—she's just a means, a way to get to an end, a stage in someone's master plan. Perhaps most degradingly, she compares herself to a cow in calf. No one wants to think of themselves as a big smelly pregnant cow—but our speaker dares to. Why? Probably to illustrate just how used she feels, as if she's livestock rather than a human being.
I've eaten a bag of green apples, (8)
The speaker sounds a little guilty in this line, which reminds us of the female famous for eating fruit that she wasn't supposed to—Eve. And remember, part of Eve's punishment was the pain of childbirth.