To slather, or not to slather—that is the question that has been roiling doctors, scientists, and new parents recently. And a new ruling by a doctor’s group stands to muck up the debate further.

Amid the birth of microbiome research, some scientists have advocated for smearing bacteria-laden vaginal secretions on any newborns who missed out—namely those born via Caesarian section. Scientists keenly hypothesize that such a gooey glaze can “seed” a more-or-less sterile infant with life-long microbial companions. These wee chums may help train an infant's immune system and dodge issues like allergies and asthma later in life. Several studies have indeed found correlations between C-section deliveries and higher risks of those conditions.

With that, motherly coatings caught on. More and more parents are now requesting “vaginal seeding” for babies delivered via C-section. But infectious disease experts, pediatricians, obstetricians, and gynecologists have pushed back, questioning the safety and noting that there’s no evidence of a health benefit.

In the latest turn to the controversy, the American College of Obstetricians and Gynecologists’ (ACOG) Committee on Obstetric Practice issued a November opinion firmly wiping up the slimy idea.

In its opinion, the committee said it:

“…does not recommend or encourage vaginal seeding outside of the context of an institutional review board-approved research protocol, and it is recommended that vaginal seeding otherwise not be performed until adequate data regarding the safety and benefit of the process become available.”

In a cleansing argument, the committee noted that studies linking the mode of delivery to allergies, asthma, and other disease risks are just correlative—they can’t say that the C-section caused the higher risks or the conditions. Despite the long-noticed link, there’s still no definitive data proving that the microbiome or various delivery methods cause the conditions. Also complicating the debate is the fact that not all C-sections are the same and that babies have varying degrees of exposure to vaginal microbes during them. C-sections may occur with or without a woman going into labor and breaking the amniotic sac, for instance.

Murky microbes

The few studies we do have on infant microbiomes provide no clear answers on the significance of an early “seeding” for health. A 2016 review looking at the patterns of microbial communities in the guts of infants in their first year found that C-section babies did show differences in the first three months. However, those differences disappeared by six months. Similarly, a small study of 18 babies also published last year found that vaginal seeding could eliminate microbial differences between vaginally and C-section delivered babies. But the study only looked at the infants' microbiomes in that first month, and the health effects—if any—are unknown.

Moreover, there is data suggesting that environmental factors—such as exposure to dust and fish oil—during early childhood do play a role in the development of asthma and allergies.

The most concerning thing about vaginal seeding, the committee argues, is the potential for transmitting pathogens, such as herpes simplex virus, human papilloma virus (HPV), group B streptococci, and Neisseria gonorrhea. The concern is underscored by recent reports from the Centers for Disease Control and Prevention that rates of sexually transmitted infections are at an all-time high . If a woman insists on the seeding, the committee recommended she be thoroughly tested and informed of the risks—as well as discouraged.

Still, microbiome researchers have been unmoved by such arguments. In an editorial in The Scientist, foremost microbiome researchers Rob Knight and Jack Gilbert concluded:

"To be clear, if a mother tests positive for any pathogens that could harm her child, we agree that clinicians should not perform vaginal seeding… But based on the evidence to date that your child’s microbiome at birth is important and modifiable, we think that parents should make up their own minds how much evidence is enough given the evolutionarily sound logic and clear health advantages of vaginal birth over C-section."

Knight has previously publicly acknowledged that he vaginally seeded his own child in 2010 following his partner’s unplanned C-section delivery. “Most ideas in science are wrong,” he wrote. “But, as my personal experience has shown, at some point it’s necessary to decide what you’re going to do before all the evidence is in.”

The one thing everyone agrees we should do is conduct more research. In its closing, the ACOG committee noted that “the paucity of data on this subject supports the need for additional research on the safety and benefit of vaginal seeding.”