The Human Microbiome and Pregnancy, Fertility, Infertility

What is the human microbiome?

We are not the only inhabitants of our bodies.  The human microbiome is the full array of microorganisms (the microbiota) that live on and in humans and, more specifically, the collection of microbial genomes that contribute to the broader genetic portrait, or metagenome, of a human. The microbiome is defined as all the bacteria, viruses, fungi, archaea, and eukaryotes that inhabit the human body. The human microbiota consists of the 10-100 trillion symbiotic microbial cells harbored by each person, primarily bacteria in the gut; the human microbiome consists of the genes these cells harbor and that help maintain health from head to toe.  The human microbiome weighs as much as 5 lb. Those that inhabit the vagina are important for reproductive health, healthy fetal development, and may even influence the child’s risk for eczema and allergies during infancy.

A great deal of study is underway to learn more about the vaginal, gut, and uterine microbiome (collection of bacteria, fungi, yeast, and other microbial inhabitants) in fertility and pregnancy.  When an overgrowth of one species outnumbers others, medical conditions such as bacterial vaginosis (BV) and urinary tract infections (UTIs) can develop.  Recent studies indicate an imbalance of microbial species (dysbiosis) contributes to the risk factor for dangerous inflammation during pregnancy which can lead to premature delivery and other complications of pregnancy and childbirth.

The vaginal microbiome and fertility

The vast majority of the human microbiome is in the digestive system.  The different species of microbes and how many of each there are depend on a person’s diet, state of health, drugs being used, lifestyle (urban or rural, developed or undeveloped nation), gender, race, age, and other demographic factors.  The diversity from person to person is immense.

This is not the case with the vaginal microbiome, however.  The composition of the vaginal microbiome is very similar in all women of reproductive age, regardless of other demographic factors.  This similarity has intrigued Dr. Gregor Reid for 33 years, since early in his career as a microbiome and probiotics scientist.

The uterine microbiome and fertility

Recent studies have shown not only that the vagina has a specific microbiome, but the uterus, which previously was thought to be sterile, does have a microbiome too.

Subclinical endometritis, infection of the endometrium without symptoms, is known to affect pregnancy rates. Failure of implantation might be explained, in part, by alteration in the uterine microbiome in response to inflammation, leading to the development of a progesterone-resistant endometrium.

A pilot study published in The Australian and New Zealand Journal of Obstetrics and Gynaecology has identified trends in the microbiome of the female reproductive tract in women experiencing infertility. The study examined the reproductive tract microbiota in two groups of women – those with a history of infertility, who were undergoing IVF treatment, and those with a history of fertility. The study found that infertile women more often had the bacteria Ureaplasma in the vagina and Gardnerella in the cervix.

The maternal microbiome and pregnancy outcome

The microbiome, not only from the vagina but also other parts such as the placenta, and the mouth is thought to play a role in the maintenance of a healthy pregnancy and thus may either contribute to or protect from preterm birth. There are now many studies that have shown how an imbalance in the vaginal microbiome and spontaneous preterm labor are related. 

The role of Lactobacilli

Reid, now affiliated with the Lawson Health Research Institute and the University of Western Ontario, both in London, Ontario, is hard at work to develop a lactobacilli-based treatment for BV and UTI that doesn’t require antibiotics.  There are several strains of the lactobacilli bacterium that make up a significant part of the vaginal microbiome.  When harmful or foreign microbes (pathogens) crowd out the lactobacilli colony, infection occurs.

Since lactobacilli are so abundant in the vaginal microbiome and this microbiome is so similar in all women, Reid feels it must be instrumental to reproductive health, pregnancy, and fetal development.  Babies are exposed to the maternal microbiome during gestation and vaginal delivery perhaps to help start their own microbiomes and immune-system development.  Lactobacilli are present in breast milk, too.

“To fully understand female health, reproduction, and infant development, we need to better understand lactobacilli,” said Reid.  He finds the study of the vaginal microbiome tricky, however, due to impossible-to-control factors that include the menstrual cycle, sexual activity, immune response, and the use of personal hygiene products that include douches.

Lactobacilli’s influence postpartum and during infancy

A team of researchers in New Zealand recently announced the launch of a study to explore what effect the probiotic Lactobacillus rhamnosus HN001 has on pregnancy, postpartum health, and infant health.  The researchers will enroll 400 pregnant women during their 14th to 16th week of gestation.  All women will be expecting babies at high risk for eczema and allergies.

Some of the women will be given Lactobacillus supplements to take until they give birth.  If they breastfeed, they will continue supplementation for six months postpartum.  The other women will take placebos.

The main goal of the study is to measure the effect of the probiotic supplements on the child’s development of allergies or eczema during its first year.  Other outcomes that will be monitored include:

  • Development of gestational diabetes.
  • Development of BV and the presence of a particular Streptococcus bacterium at 35 to 37 weeks gestation.
  • Maternal weight gain.
  • Postpartum depression and anxiety.
  • Infant birth weight.
  • Preterm birth.
  • Rate of cesarean deliveries.

The researchers will test samples of maternal and infant fecal samples, maternal and cord blood samples, and breast milk.  Infant skin and cheek swabs will also be tested.  The researchers hope to discover if treating only the mother during pregnancy and breastfeeding will reduce the child’s risk for allergies and eczema without treating the baby itself.

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