What is a preterm birth?
On average, a pregnancy lasts 40-weeks. A preterm or premature birth is a birth that occurs before the start of the 37th week of pregnancy.
When a preterm birth occurs without medical interventions it’s known as spontaneous preterm birth. Sometimes, a preterm birth occurs when a provider needs to induce birth because of complications.
For example, preeclampsia, a pregnancy complication characterized by high blood pressure and organ damage, may necessitate preterm birth .
In 2019, the total rate of preterm birth in the US was 10.23% . This rate varies by race, being the highest for Black women (14.39%) and the lowest for Asian women (8.72%).
What causes a preterm birth?
Most preterm births occur spontaneously. The exact cause is often unclear , but there are some things that may increase your risk of going into labor before your due date. Risk factors for preterm birth include:
- Carrying twins, triplets, or other multiples
- A history of preterm birth
- Getting pregnant through in vitro fertilization (IVF)
- Physical trauma and stressful life events
- Chronic conditions like diabetes, obesity, high blood pressure , 
Another known risk factor for preterm birth are sexually transmitted infections (STIs).
For example, a vaginal infection with Trichomonas vaginalis during pregnancy increases the risk of preterm birth. What’s more, babies born to infected mothers are more likely to have low birth weight .
Other STIs that increase the risk of preterm birth include:
- Gonorrhea, an infection caused by Neisseria gonorrhoeae bacteria. This STI has been associated with preterm birth, premature water breaking, low birth weight, and other pregnancy complications , .
- Chlamydia, caused by Chlamydia trachomatis bacteria. Untreated, this infection during pregnancy has been associated with preterm birth –.
- Syphilis. Depending on how long a pregnant woman has had syphilis (an infection caused by Treponema pallidum subsp. pallidum bacteria), she may have an increased risk of preterm birth. Syphilis can also lead to other serious consequences for a mother’s baby , .
And still, often early labor and delivery happens without any risk factors involved.
Preterm birth symptoms
There are certain symptoms that may indicate premature labor:
- Regular contractions
- A constant and dull backache
- Mild abdominal cramps
- A sensation of pelvic or lower abdominal pressure
- Vaginal spotting or light bleeding
- Water breaking
- Changes in vaginal discharge 
If you are far from your due date and suspect you have preterm birth symptoms, contact your provider right away.
It could be a false alarm, but it’s always better to ask. Your provider may run a series of tests including a pelvic exam, an ultrasound, uterine monitoring, and other lab tests to identify if you’re in preterm labor.
Preterm birth and its associated complications for mom and baby
Preterm birth can mean a variety of short and long-term health complications for your baby. Complications tend to be more severe the more premature the birth.
After birth, there may be problems with breathing, heart or brain function, temperature regulation, blood disorders like anemia or jaundice, gastrointestinal function, metabolism, or issues involving your baby’s immune system.
Over time, babies born prematurely have an increased risk for sudden infant death syndrome (SIDS), cerebral palsy, developmental disabilities, issues with vision and hearing, and dental problems .
One study found that preterm babies have a slightly higher risk of developing type 1 diabetes in the future .
Mothers who give birth preterm are more likely to experience issues like anxiety, postpartum depression, post-traumatic stress disorder, and problems connecting with their baby .
How a healthy vaginal microbiome protects against preterm birth
We know that bacteria that cause STIs can increase the risk of preterm birth. But what about other bacteria, like those that can cause bacterial vaginosis?
Scientists have tried to identify if certain bacterial species of the vaginal microbiome, often associated with dysbiosis or an unhealthy balance of bacteria, can directly increase the risk of preterm birth.
Some bacteria that have been associated with preterm birth are Sneathia sanguinegens, Megasphaera, and Gardnerella , . But, some studies haven’t found such direct associations –.
Looking at all the studies performed up to this day, there are two main things we can conclude:
- High levels of Lactobacillus species (except for L. iners) have a protective effect and reduce the risk of preterm birth. This is great!
- On the other hand, women with lower levels of non-iners Lactobacillus species or with a Community State Type (CST) 4 are at higher risk of preterm labor , , –.
For example, one study found that pregnant women with CST 4 were roughly 7 - 8 times more likely to give birth prematurely compared to women with a non-iners Lactobacillus-dominated vaginal community . Another study found that pregnant women with CST 4 were over 3 more likely to have recurrent spontaneous preterm birth than women with a Lactobacillus-dominated vaginal community .
About one third of preterm births happen after the preterm prelabor rupture of membranes (PPROM), also known as water breaking. This gives an opportunity for microbes in the vagina to go up into the amniotic sac, where they may cause infection.
What’s noteworthy here is that scientists have found that a high number of women who experience water breaking also have a vaginal microbiome with low or moderate numbers of Lactobacillus.
Remember, Lactobacillus are friendly and keep the vaginal microbiome healthy.
For example, the risk of water breaking among pregnant women with a vaginal community with less than 75% Lactobacillus was 2.34-2.54 times higher than the risk for women with a Lactobacillus-dominated vaginal community , .
But why do low levels of Lactobacillus increase the risk of preterm birth? Scientists have just begun to unravel this.
A study found that compared to women who delivered at term, women who delivered preterm had low levels of non-iners Lactobacillus and this correlated with higher levels of certain immune molecules that recognize and bind microbes. This led to increased levels of inflammation .
So the proposed mechanism is that a vaginal microbiome low in non-iners Lactobacillus can’t properly contain an inflammatory response.
The microbes that live in your mouth may also have something to do with preterm birth
Pregnant women are more prone to suffer from periodontal disease. Scientists have two theories on how this may be connected to premature labor:
- The microbes that cause periodontitis may travel through the bloodstream into the placenta and cause infection that leads to preterm birth.
- Proinflammatory molecules produced in the mouth can go into the bloodstream and into the placenta, causing inflammation that leads to preterm birth.
One study found that the risk of spontaneous preterm birth in pregnant women with moderate to severe periodontitis was twice the risk of spontaneous premature birth among healthy pregnant women .
Then comes the obvious question: Does treating periodontitis decrease the risk of premature birth?
Well, scientists are not convinced.
One study found that pregnant women who didn’t receive periodontal treatment during pregnancy were 13.5 times more likely to have preterm birth than women that received the treatment . That’s a pretty high risk, right?
However, a major pitfall of that study was not accounting for other variables that may have influenced the risk. That is, confounding factors. Besides, other studies have found that the risk of preterm birth doesn’t decrease when periodontal disease is treated –.
Clearly more studies are needed on this matter.
The gut microbiome and preterm birth
The connection between the gut microbiome and preterm birth is a recent area of research. Just a few studies have looked at the possible associations.
Related to the possible role of oral microbes in preterm birth, a small study found that women who had premature labor had increased numbers of oral bacteria in the gut, like Streptococcus, Porphyromonas, and Prevotella . Another study found that women who had premature labor had lower numbers of Clostridium and Bacteroides species, which are common gut bacteria .
5 tips to protect against preterm birth
Although the cause of preterm birth is often unknown and there are many factors involved, there are some things that may help reduce the risk.
- Consider taking folic acid while trying to conceive. If you aren't pregnant yet but plan to be soon, you can begin taking folic acid three months before conception. In addition to its critical role in early neurological development of a baby, folic acid may also help reduce the risk of preterm birth. Studies have reported a risk reduction of 8-14% –.
- Make sure you are taking adequate amounts of calcium during pregnancy. Getting enough calcium during pregnancy can help to reduce the risk of preterm birth –. This may come from diet and/or supplements, so it’s a good idea to check whether you are receiving enough calcium with your provider.
- Check your vaginal microbiome. Several studies show that women with a CST 4 or bacterial vaginosis have a higher rate of preterm birth , –. Unfortunately, there are no well-designed clinical trials evaluating if a shift in vaginal microbial composition reduces the risk of preterm labor. Some clinical trials have looked at the effect of treating bacterial vaginosis with antibiotics, and have found no reduction in the risk –.
- Wash only with water. One study found that women who used a vaginal gel before or during pregnancy had 2.3-3.4 times higher risk of preterm birth . Here are some tips for vaginal health that will help you take good care of your vaginal microbiome.
- Probiotics may help delay birth after water breaking. If water breaks before week 34, birth may need to be delayed. This is accomplished by corticoid and antibiotic treatment. Two studies found that women who also received vaginal probiotics (inserted in the vagina) had a larger latency period, which resulted in increased gestational age at birth , . A third study didn’t find that probiotics had any effects in gestational age, but the babies from moms who received probiotics had better health outcomes than those from moms who only received conventional treatment . Despite these encouraging results, research on vaginal probiotics and preterm birth is still in progress. Not all vaginal probiotics may have the same effect as those that were studied, and right now we can’t recommend one for this particular situation. If you find yourself in this scenario, always ask your provider before using any vaginal probiotics.