Lines 5-9 Summary

Get out the microscope, because we’re going through this poem line-by-line.

This loaf's big with its yeasty rising.

This line moves us back into some of the more obvious metaphors for pregnancy. It's easy to picture a pregnant woman as a loaf of bread rising, what with the slow swelling of her belly as a child grows inside.
And even though we love freshly baked bread just as much as the next person, there's something not very appetizing about thinking about a baby along the same lines as bread fresh out of the oven.
In fact, it sounds a little sad and cynical the way that the speaker thinks of pregnant women. No pregnant woman that we know wants to think of herself as a loaf of bread. Or a melon on skinny legs, either, for that matter.

Money's new-minted in this fat purse.

We're back to money again (like the expensive ivory and fine timbers from line 4). The idea that the money is "new-minted" is referring to freshly printed money. We can think of this literally— as printed dollars or coins that are brand new, straight from the US mint. We can also think of it metaphorically, in which case the pregnant woman's baby would be the new money. Yeah—that's not a flattering comparison.
The "fat purse" is another thing that can be taken on a few levels. First, there's the idea that someone who has a "fat purse" is really rich. Their purse is fat because of all the money they've presumably stuffed into it.
An alternate idea is that the woman is simply the metaphorical purse for the metaphorical money. The line says literally that we're talking about money in a purse, but we're supposed to solve the riddle and figure out that it's referring to a baby in a woman's body. This interpretation is aided by the use of the word "purse" as an innuendo for the female sexual organ, especially for prostitutes. Again with the flattering comparisons. Eesh.
Also, check out the sounds in this line. There's some awesome alliteration, or repetition of consonant sounds, in the phrase "money's new-minted."

I'm a means, a stage, a cow in calf.

For the first time since the first line, we see the word "I" again. This time, the meaning of the riddle is starting to become obvious.
By saying "I'm a means," the speaker is telling us that she feels as if she's being used, just a means to an end. She's just a stage in someone else's master plan—the bearer of their fruit.
Saying that she's a "cow in calf" is probably the most obvious part of the riddle, making us certain that the riddle is giving us metaphors about a pregnant woman.
But the purpose of this line is not just to solidify our guesses about the meaning of the poem. A cow that's pregnant—or, as farmers would say, in calf—is an unflattering way to think about a pregnant woman. Mean people sometimes use the word "cow" to describe women they don't like, or women they think are fat and unattractive.
There's also, to follow the theme from earlier in the poem, an economic side to the idea of a pregnant woman being a cow in calf. Cows are milked, or killed for slaughter, and sometimes calves are killed to make veal. When we picture it that way, thinking about cows and their babies on the same level as a pregnant woman is straight up messed up. Slaughter and birth are polar opposites, and yet, somehow, our speaker finds a way to connect these two events with a little trick called figurative language.

I've eaten a bag of green apples,

Again, we see the "I" in this line. Eating a whole bag of green apples in a short sitting, we could imagine, would make the speaker's stomach quite round, so this is another metaphor for the large belly of pregnancy.
There's also a weird sense of guilt here. This admission to eating the bag of green apples could mean that she felt that she's overindulged, and that is the cause that she's become this huge, ponderous pregnant woman.
We can also imagine that these green apples would be quite sour. Eating a bag of green apples would be a lot more difficult on the taste buds than eating a bag of red apples.
This image also makes us think of some other famous females, like Snow White and Eve, who both took bites out of apples and, well, we know how it turned out for those two.

Boarded the train there's no getting off.

This line, which is enjambed with the previous line, seems to imply that boarding the train is either along the same thought-line, or as a result of eating the bag of green apples.
This line shows us that our speaker knows that, so to speak, once she's gotten pregnant, she's boarded a train that she can't get off of—her life is forever changed and there's no going back.
For other women, who perhaps choose another path (abortion), or have it thrust upon them (miscarriage), there's a chance things may change, but this woman knows that she is stuck with this baby, this life as a mother, for good.
This train stretches for at least nine months, and, if the baby is not given up for adoption, for the rest of the mother's life.