Miscarriage

What is a miscarriage?

A miscarriage is the loss of a pregnancy. Depending on your healthcare system, this may be defined as before the 20th or 24th week of gestation [1, 2]. If the baby is lost later in the pregnancy or during childbirth, it is considered a stillbirth [3]. Miscarriage is often the result of the fetus or embryo not developing normally. An "early miscarriage" is when the loss occurs before 12 weeks and a “late miscarriage” is when the miscarriage occurs at week 12 or later. [3]. 

Most women can go on to have a successful pregnancy following a previous miscarriage. However, it is possible to have “recurrent miscarriages”, defined as three or more consecutive miscarriages [3]. Most women are able to have healthy subsequent pregnancies, even in the case of recurrent miscarriages.

Maternal age is the greatest risk factor for miscarriage, but other risk factors include paternal age, obesity and drug or alcohol use [3]. There are also a number of long-term health conditions that may increase your risk.

How common are miscarriages?

It is estimated that 26% of pregnancies result in miscarriage [4]. Early miscarriages are more common than late miscarriages, with many occurring before the pregnancy is detected. 

Chromosomal abnormalities (problems in the genetic material of the fetus) are the most common cause of miscarriage, accounting for around half of early miscarriages [3]. 

Infection is also a significant contributor to miscarriage. Preventable infection accounts for up to 15% of early miscarriages and 66% of late miscarriages [3].

How are miscarriages linked with the vaginal microbiome?

The majority of late miscarriages are caused by harmful microbes ascending from the vagina through the cervical canal into the uterus. Here the microbes can infect the amniotic fluid and the developing fetus [5]. 

Your vaginal microbiome is the community of microbes living in your vagina. A healthy vaginal microbiome is typically dominated by a type of good bacteria called Lactobacillus, and it can protect you from harmful microbes. 

However, this is a carefully balanced ecosystem. When the balance is disrupted it can lead to conditions like bacterial vaginosis (BV), characterized by low levels of Lactobacillus and high levels of other anaerobic bacteria. Multiple studies have identified a link between BV and miscarriage [3] and antibiotic treatment of BV has been shown to reduce the risk of late miscarriage [6]. 

Infection with specific microbes also puts you at higher risk. Chlamydia trachomatis, the most common sexually transmitted infection (STI) in developed countries, is associated with miscarriage [3]. Having a healthy vaginal microbiome is thought to protect you from Chlamydia infection [7]. 

Other bacteria associated with miscarriage include Ureaplasma species, Mycoplasma hominis and group B streptococcus (GBS; Streptococcus agalactiae) [5]. These species are also often found in healthy vaginas without causing any problems [8, 9], so more research into the vaginal microbiome is needed to help us understand what causes these bacteria to turn bad.

Infection is also associated with recurrent miscarriage in cases of chronic endometritis (CE) [12]. CE is the long-term inflammation of the endometrium (the lining of the uterus) as a result of an immune response to the microbes in the endometrium. CE is often asymptomatic or overlooked due to mild symptoms, therefore its prevalence in the general population is unclear. In people who have experienced recurrent miscarriage the rate of CE could be as high as 67% [11]. 

The successful treatment of CE with antibiotics has been shown to improve pregnancy outcomes in cases of multiple miscarriages [12]. Evidence suggests that intervention at the vaginal microbiome level could also be beneficial. The vaginal microbiome has been shown to influence the composition of the uterine microbiome and overall uterine health [13], and microbes in the vaginas of people with CE have been found to be more diverse and have less Lactobacillus than people without CE [14]. 

What are the diagnostic and treatment options?

Common symptoms of miscarriage [15] include vaginal bleeding and severe abdominal pain or cramping. If you think you are having a miscarriage it is important to seek medical help to be safe. Your doctor or midwife can carry out tests to confirm the loss of pregnancy and, in some cases, further treatment may be required to remove retained pregnancy tissue. 

You should be offered pain relief and psychological support for yourself and your partner. While physical recovery might only take a few hours or days, psychological recovery from emotional distress will vary. 

To identify the cause of a miscarriage you may be offered tests for infection, chromosomal abnormalities, underlying health conditions or abnormalities in the shape of your uterus.  

However, testing to identify the cause is typically not typically offered until someone has experienced three or more miscarriages, so the cause is often not known.

What can I do to prevent a miscarriage?

You should discuss your medical history and the risk factors for miscarriage with your doctor or midwife to see if there are any interventions that could improve your chance of a successful pregnancy.

If you have had a miscarriage and you suspect it could have been caused by an infection, you should seek testing and treatment for the infection. Then, if you get pregnant again, you can be tested throughout the pregnancy and treated accordingly.

Where can I get more information about miscarriages?‍

While your healthcare can help guide you in navigating the complexities around miscarriage, Planned Parenthood also offers additional information.