It's based on an observation that you quoted in the following twitter thread:
I extracted the relevant image for discussion:
View attachment 46796
The second trimester of pregnancy goes up to the 26th week, and reported spontaneous abortions are at less than 20 weeks of pregnancy. The vaccine campaign started on December 2020.
If analysis of the entire study rules out the above assumption ("127 had vax in Trimester 1 and Trimester 2" from the v-safe Pregnancy Registry), then, carry on, nothing to see here. But if the above assumption holds true, even after analysis of the entire study's results, then that is where you get your super high rate of miscarriages in the first 20 weeks.