It's this study: https://www.nejm.org/doi/full/10.1056/nejmoa2104983. Look at the data not their conclusions. 104 spontaneous abortions and only. They do a good job of disguising it but logically anyone who got vaccinated during third trimester should be excluded from the denominator for determining percentage of miscarriage because in the third trimester a failed pregnancy is classified as a still birth. So, if we remove those 700 people, the math becomes 104/(827- 700)=83% Here's a the relevant text from the article:

Among 827 participants who had a completed pregnancy, the pregnancy resulted in a live birth in 712 (86.1%), in a spontaneous abortion in 104 (12.6%), in stillbirth in 1 (0.1%), and in other outcomes (induced abortion and ectopic pregnancy) in 10 (1.2%). A total of 96 of 104 spontaneous abortions (92.3%) occurred before 13 weeks of gestation (Table 4), and 700 of 712 pregnancies that resulted in a live birth (98.3%) were among persons who received their first eligible vaccine dose in the third trimester.