The timing is a very obvious reason to pin it on the vaccine because that's what was changing at that time. Omicron emerged later (end of 2021) and is much milder.

But yes, one way to answer that question definitively would be to do a cohort study on conception rates. Split the data by vaccine status. This can't be done because public health agencies systematically refuse to do this sort of study, nor do they expose the necessary raw data. They don't want anyone being able to answer these sorts of questions. Most data that is available has to be forced out via FOIA requests and the courts.

Are we looking at the same graph? Vaccinations were well past the inflection point in August in the data you are responding to, so the vaccine affect would have already happened if we make the bad simplifying assumption that the population is homogeneous. But Delta was drastically increasing infections in August, leading to a proportional much smaller increase in vaccination then.

Well, for me there's no real need to try and divine it from some rough and ready study. I already know that the vaccines drive this because so many of my girlfriend's social groups reported disrupted cycles immediately after taking the shots, and for some the doctors said specifically it seemed to be vaccine caused. Seemed for them actually mostly that the periods vanished, rather than unexpected bleeding.

Except they would have shown up earlier if it was COVID-19 infections driving this rather than vaccinations driving it. Timing matters here.