Welcome to the second installment of what I expect will be a regular feature. In this feature, I peruse academic journals and report back. In these posts I’ll try to surface interesting results that were not breathlessly covered in the media but that, nevertheless, may be of interest to this audience. A bonus is that many of these journal articles are actually a lot more methodologically sound than much of what is in the media.

Last time, we reviewed papers from JAMA Pediatrics. This time, I’m focused on pregnancy, so I turned to the American Journal of Obstetrics and Gynecology (impact factor, a high 8.661).  

Group vs. Individual Prenatal Care

The traditional model of prenatal care is one patient with one provider, in semi-frequent but quite short visits. A typical prenatal visit spends considerable time on measurement and checking the baby with ultrasound or Doppler. What it doesn’t always have is a lot of time for discussion, and the environment can be a difficult one in which to ask questions.

This is one of the issues that group prenatal care has aimed to address. Group prenatal care — the most well-known program is called CenteringPregnancy — has a different approach. Patients come for a prenatal visit that is 90 minutes to two hours long, along with 8 to 10 other people at a similar stage of pregnancy. Everyone has their vitals checked as usual, but then most of the visit is taken up with discussion between patients, providers, and support staff.

This group prenatal care model has some significant cost advantages, but a primary reason many people support it is early evidence suggesting it might lower preterm birth rates and, more importantly, reduce racial disparities in preterm birth and other outcomes. The U.S. has extremely high preterm birth rates in general relative to peer nations; the rates for Black women are much higher still.

This is background to the first paper I focused on, which is the largest randomized trial to evaluate the impact of group prenatal care on preterm birth and low birth weight. The trial included 2,350 participants, who were split in half into group or individual prenatal care. Because this split was done randomly, we can be confident about inferring causal effects. The trial was designed to over-sample Black participants, so it would be possible to infer different effects by race (if present).

The results of this trial are a bit disappointing. Group prenatal care did not lower the rate of preterm birth either overall or within Black participants. The racial difference in preterm birth rates was lower in group prenatal care than individual, but that was driven by higher preterm birth rates among white participants in the group care. There are better outcomes for some women if they attend more prenatal visits, but that’s not a randomized component of the study.

This doesn’t mean group prenatal care isn’t a good idea; the study was ultimately underpowered statistically, given a lower-than-expected preterm birth rate, and it didn’t look at a variety of other outcomes (like patient satisfaction) that are often covered. The research opens as many questions as it closes. But if we were looking for a silver bullet for racial inequity in birth outcomes, this suggests group prenatal care isn’t it. We need to keep working.

COVID Vaccines in Pregnancy and Infant Illness

COVID may not be occupying as much brain space for many people, but because of publication lags, it’s occupying substantial space in journals. The most interesting of the many recent COVID papers, to me, was one about the level of antibodies in infants following maternal vaccination in pregnancy.

In this study (really, a research letter), the authors report data from 40 infants in Israel whose mothers were vaccinated with Pfizer during the second trimester of pregnancy. Antibody levels in infants were measured in cord blood and then in a blood draw later in infancy (sometime between 15 and 26 weeks of age).

At birth, the antibody levels in infants are very high. They decline over time but remain substantial (on average) even through five or six months. I’ve included the original graph below. The average antibody level at birth is 2,790 “arbitrary units” per milliliter (AU/mL). The authors report decay as a percentage of the initial level; any level above 50 is considered to be a positive amount of antibodies, meaning anything on this graph above about 2%. Overall, infant protection is sustained through about six months. The rates are higher for breastfed than non-breastfed infants, though both have significant protection here.

Bottom line: COVID vaccination during the second trimester of pregnancy provides antibody protection through six months.

Predicting Recurrent Preterm Birth

Finally, we have two more papers on preterm birth. Both of these papers deal with an extremely important question: Can a recurrence of preterm birth be predicted? Preterm birth in general is poorly understood. Although there are some known behavioral risk factors (e.g. smoking), in most cases a first preterm birth is unexpected. Having had one preterm birth is predictive of having another, but not perfectly. Identifying what factors correlate with a recurrence of preterm birth is both useful in terms of identifying who is at risk for recurrence and may help us better understand what drives preterm birth overall.

In the first of these papers, the authors ask whether recurrent preterm birth can be predicted based on studying the placenta. They find that “high-grade chronic inflammation” of the placenta increases the risk of recurrent preterm birth by about 37%. This effect is actually fairly small, and only marginally statistically significant. Overall, it’s probably too underwhelming to be something that would be added to any screening, especially since it’s quite involved to measure the placenta.

Potentially more promising is this paper, which shows that a short cervical length within the first 24 hours after birth predicted a recurrence of preterm birth. The authors find that this information is actually more predictive than a spontaneous preterm birth (although you could get higher predictive value from combining the two). This seems much more practically important to me, since measuring the length of the cervix is not complicated to do.

Why not in the media? 

It’s worth asking yourself at the end of these segments: Why didn’t I see these papers covered in the media? In these cases, I think the answers vary.

The first paper — on group prenatal care — probably didn’t get covered because the result wasn’t significant. This is perhaps useful to remember as we process evidence reporting in general. We are much less likely to see studies covered in the media if the results aren’t statistically significant. Even if, as in this case, the topic is important and something we want to know, it’s just not as media-friendly.

The second paper, on COVID vaccines — I’m not sure why that didn’t get more attention. Certainly I spend a lot of time fielding questions about the value of COVID vaccination in pregnancy. So this seems like something many people would be interested in. I suspect the media is a bit tired of covering COVID.

These last two papers are far too in-the-weeds of obstetrics to warrant wide coverage. But I’d argue they are really where the sausage of medicine gets made. A problem like understanding recurrent preterm birth is not going to be cracked by one magical study with all the answers. It’s something we’ll slowly figure out, with studies that give us small clues and then that we piece together into an overall understanding. The idea of measuring postpartum cervical length and using that to help women make decisions about their next pregnancy — that’s powerful and important, even if it is not precisely sexy.

That’s all for today. If you have a particular research area (or a specific journal!) you’d like to see next time, weigh in in the comments!