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
- Preeclampsia, a condition where a mom develops high blood pressure
- Chorioamnionitis, an infection of the amniotic fluid and the membranes surrounding it , .
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?
Looking at all the studies performed up to this day, there are two main things we can conclude:
- High levels of Lactobacillus species, especially Lactobacillus crispatus, have a protective effect and reduce the risk of preterm birth. This is great!
- On the other hand, women with lower levels of Lactobacillus species or with a Community State Type (CST) 4 are at higher risk of preterm labor –.
For example, pregnant women with CST 4 are roughly 3 - 8 times more likely to give birth prematurely or to have recurrent spontaneous preterm birth than women with a Lactobacillus-dominated vaginal community  .
There are also other bacteria that have been associated with increased risk of preterm birth. Examples are Fannyhessea vaginae, Prevotella bivia, and Sneathia amnii.
This risk is higher when the vaginal community is not dominated by Lactobacillus.
As it turns out, 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 2.3-2.5 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 gum disease (periodontitis). Scientists have two theories on how this may be connected to premature labor:
- The bacteria 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.
Different studies have found that compared to pregnant women with healthy gums, the risk of spontaneous preterm birth in those with periodontitis is higher –.
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. Although we can't say for sure that periodontitis treatment reduces the risk of preterm birth, we think it's worth being cautious and taking good care of your oral health before and during pregnancy.
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 .
4 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 visiting your dentist regularly before and during pregnancy. Hormonal changes during pregnancy may make you more prone to gum disease and cavities. Gum disease starts as gingivitis and can progress to periodontitis if not treated, which may increase the risk of preterm birth –.
- Consider taking folic acid as soon as you begin 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% –.
- Check your vaginal microbiome. Several studies show that women with a CST 4, low levels of Lactobacillus, 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 –. If your vaginal community is not in an optimal state, it could be worth trying some probiotics or vaginal suppositories containing Lactobacillus crispatus to help crowd out bacteria associated with a higher risk of preterm birth.
- Be cautious about vaginal products such as gels and douches. One study found that women who used an over-the-counter vaginal gel such as Lactacyd before or during pregnancy had 2.3-3.4 times higher risk of preterm birth . For douching, the findings are somewhat controversial. Some studies have found douching increases the risk of preterm birth and other studies have found it doesn’t –. Since some vaginal gels and douches contain chemicals that may disrupt your vaginal microbiome, we think it’s better to stay away from those if possible. Here are some tips for vaginal health that will help you take good care of your vaginal microbiome.