# Did 79% of pregnancies in an early COVID-19 vaccine study result in loss?

I just came across a paper published last year concerning the safety of covid 19 vaccines in pregnant women. I read the following sentence in the abstract:

Among 3958 participants enrolled in the v-safe pregnancy registry, 827 had a completed pregnancy

Does this mean that 79% ((827-3958)/3958) of women had a miscarriage in the study? I could be wrong though. I wanted to see if others have come across this study, or can help me properly interpret the results.

• the specific part in the research article I am confused about, is in the results where it states, "Among 3958 participants enrolled in the v-safe pregnancy registry, 827 had a completed pregnancy." From this it seems that only 20% of the participants carried a pregnancy to term? Which seems like a very low number. Commented Jan 9, 2023 at 13:20
• @IanCampbell can you help me properly interpret the 3958 patients that were pregnant, 827 had a completed pregnancy? That's the part I am confused about Commented Jan 9, 2023 at 13:53
• I'm not sure where you came up with 82%. I edited the question with the quote and the math that comes to 79%. Please clarify if I have misinterpreted. Commented Jan 9, 2023 at 15:10
• @IanCampbell thank you for editing the question for me. This is my first time using this website. Commented Jan 9, 2023 at 16:11
• @IanCampbell i dont know why i originally said 82 lol. I did (827/3958) * 100 to get the 79. Thanks again for editing the question for me. Commented Jan 9, 2023 at 16:15

No, please refer to the rest of the sentence:

Among 3958 participants enrolled in the v-safe pregnancy registry, 827 had a completed pregnancy, of which 115 (13.9%) were pregnancy losses and 712 (86.1%) were live births

To further clarify, the abstract of the article notes:

From December 14, 2020, to February 28, 2021, we used data from the “v-safe after vaccination health checker” surveillance system

That is a total of 76 days.

Median human gestation is approximately 268 days. Thus, less than a third of a human gestation was covered by the study period.

Therefore it is perfectly reasonable that only 827 of 3958 (21%) participants completed their pregnancy during the study period. Presumably, some of the participants didn't fill out the survey that lets the staff know about the outcome.

Of the 827 who did have their pregnancy complete, 115 (13.9%) ended in a loss. This is consistent with previous estimates of pregnancy loss in the United States (Wilcox et al 1988. PMID 3393170).

• In a study with this design, you'd expect far higher losses as a % than over a full-term pregnancy, simply because there is no possible way a pregnancy can end in a healthy birth for the majority of pregnancies in the study. Commented Jan 9, 2023 at 15:30
• I suspect there is also a moderating effect. Pregnant persons who are early in their gestation (and thus can't possibly result in a live birth) are less likely to themselves know they are pregnant and report it to the study staff. On the other hand, people who are nearing the end of gestation may be less likely to participate since... you know, they have other things to think about. I'm glad I'm not the one who has to try to model all this. Commented Jan 9, 2023 at 15:50
• Yep, good points. I was assuming that we aren't counting any early miscarriages at all, or 13.9% would be remarkably low. Commented Jan 9, 2023 at 15:59
• @IanCampbell thanks for the answer. I didn't think of that possibility that the other % just hadn't completed the pregnancy bc they simply didn't give birth lol. That does seem to be the simpliest answer. Thanks again, I appreciate it Commented Jan 9, 2023 at 16:21
• @BishopD: I believe this study was also the topic of a question over on Skeptics a while ago, and the conclusion was basically the same: there is nothing surprising about the fact that few pregnancies complete in 2.5 months nor is there anything surprising about the fact that a high number of pregnancies that complete in (less than) 2.5 months complete abnormally. Commented Jan 12, 2023 at 15:25