Not even close to reality.
Let your "little head" take a break from how horny the idea of a dog fucking a pregnant woman makes you, and use your "big head" for a moment instead:
Fact 1: A typical dog's knot is *ROUGHLY* size-equivalent to a tennis ball - Call it 3 inches in diameter to be generous. Regulation diameter of a tennis ball is actually 2.5-2.7 inches. This obviously (or at least, it SHOULD be obvious) ignores the fact that a dog's knot isn't actually a sphere, but for this discussion, treating it as if it were is close enough.
Fact 2: A baby is *ROUGHLY* size-equivalent to a bowling ball - Regulation diameter of a bowling ball is 8.5 inches. Like the dog's knot, a baby isn't a sphere, but again, close enough for discussion purposes.
Being able to roll a tennis ball through a piece of 4 inch PVC pipe won't make getting a bowling ball through it any easier.
That's EXACTLY the kind of comparison you're making: "I can roll a tennis ball through this pipe, so rolling a bowling ball through it will be easier."
In a word: WRONG. In two words: TOTALLY wrong.
Please, in the future, if you've got the urge to offer advice on a topic, restrict yourself to things that you actually have some knowledge of. Especially when the topic could affect someone's well-being if they try to use your foolish advice.