The Hidden Microbiome That Could Shape Pregnancy and Baby Health - Prof David MacIntyre

For years, the spotlight in microbiome research has been firmly on the gut. We’ve learned that diversity there is generally a good thing—linked to resilience, health, and better outcomes.

But what if one of the most important microbiomes in the body follows completely different rules?

In a recent conversation, Dr James McIlroy sat down with Professor David MacIntyre, Director of the Robinson Research Institute, to explore a rapidly emerging field: the female reproductive tract microbiome.

What they uncovered challenges many assumptions—and could transform how we think about pregnancy, birth, and early life health.

A Different Kind of Microbiome

When we talk about the gut microbiome, diversity is often considered a hallmark of health.

The reproductive tract? It’s the opposite.

In healthy women, this environment is typically:

• Low in diversity

• Dominated by one key group of bacteria: Lactobacillus

This might sound counterintuitive—but it’s by design.

Why low diversity works here

Certain Lactobacillus species—particularly Lactobacillus crispatus—play a protective role by:

• Producing lactic acid, keeping the environment acidic (around pH 4)

• Preventing harmful microbes from growing

• Blocking pathogens from attaching to tissue

• Producing antimicrobial compounds

In short, they create a stable, protective ecosystem.

When this balance is disrupted and diversity increases, it’s often linked to:

• Infection

• Inflammation

• Poor reproductive outcomes

A Unique Human Feature

One of the most surprising insights?

Humans are the only species known to have a Lactobacillus-dominated reproductive microbiome.

Even our closest primate relatives don’t share this feature.

That suggests this system evolved relatively recently—and likely offers a meaningful biological advantage, particularly in pregnancy and childbirth.

The Link to Preterm Birth

Preterm birth—defined as delivery before 37 weeks—affects around 1 in 10 pregnancies globally and remains a major medical challenge.

The microbiome appears to play a key role, particularly in early preterm births.

What’s happening biologically?

• In a healthy pregnancy, labour is triggered by controlled, sterile inflammation

• This process helps:

  • Break the membranes (waters)

  • Open the cervix

  • Initiate contractions

But if inflammation starts too early, it can trigger premature labour.

Where the microbiome fits in

A Lactobacillus-dominant microbiome helps:

• Keep inflammation low

• Maintain a strong mucosal barrier

• Reduce infection risk

When this balance is lost:

• The environment becomes less acidic

• Inflammatory molecules increase

• The barrier weakens

• Risk of infection—and preterm labour—rises

Once inflammation starts, it’s very difficult to switch off.

That’s why prevention is so important.

What Disrupts the Balance?

Several factors can shift the microbiome away from its protective state:

• Antibiotics (even when used for unrelated infections)

• Sexual transmission of bacteria

• Hormonal changes

• Menstruation

• Hygiene practices like douching

Even small changes in pH or environment can allow harmful bacteria to gain a foothold.

Can We Predict Risk Earlier?

One of the most exciting areas of research is using the microbiome as a predictive tool.

Scientists are working towards:

• Identifying microbial “signatures” linked to higher risk

• Measuring both:

  • Which microbes are present

  • How the body is responding to them

New technologies can now analyse a simple swab and generate a metabolic “barcode” in minutes—potentially enabling rapid screening.

The long-term vision?
Routine testing that identifies at-risk pregnancies early—before symptoms appear.

Can We Change the Microbiome?

If we can measure it, can we fix it?

1. Oral probiotics: limited impact

Despite widespread marketing, there’s little evidence that oral probiotics reliably reach or colonise the reproductive tract.

They may have indirect benefits—but they’re not an efficient delivery method.

2. Targeted vaginal probiotics

More promising is direct delivery of beneficial bacteria.

A recent clinical trial explored a vaginally delivered strain of Lactobacillus crispatus:

• Safe and well tolerated in pregnancy

• Successfully colonised the microbiome

• Showed early signs of reducing preterm birth risk

A larger, definitive trial is now underway in the UK.

3. Microbiome transplantation

Inspired by faecal transplants, researchers are exploring vaginal microbiome transplants.

Early studies show potential—but challenges remain:

• Regulation

• Standardisation

• Safety at scale

For now, this remains experimental.

The First Microbial Gift: Birth

The microbiome’s role doesn’t stop at pregnancy.

During vaginal birth, babies are exposed to their mother’s microbes—an important early “seeding” event.

Babies born via caesarean section:

• Have different early microbiomes

• May face higher risks of certain conditions (e.g. allergies)

There’s growing interest in “vaginal seeding” for C-section babies—but:

• Evidence is still emerging

• Safety concerns remain

• It’s not yet standard practice

A Shift Towards Personalised Pregnancy Care

Looking ahead, this field is moving towards a precision medicine approach.

Instead of treating all pregnancies the same, we may soon:

• Assess individual microbiome profiles

• Measure immune responses

• Identify personalised risk

• Apply targeted interventions early—or even before conception

The goal?

Not just preventing complications—but optimising pregnancy from the start.

Why This Matters

This isn’t just about microbes—it’s about long-term health.

By improving the maternal microbiome, we may:

• Reduce preterm birth

• Improve newborn outcomes

• Influence lifelong health trajectories

As Professor MacIntyre puts it, we’re on the cusp of a major shift—from reacting to problems to preventing them before they begin.

00:00 Show Returns

01:34 Why Reproductive Microbiome

05:15 Low Diversity Health

09:24 Lactobacillus Protection

12:06 Human Uniqueness

14:17 Population Differences

16:31 Oral Probiotics Myth

19:25 Vaginal Biotherapeutics Trial

22:11 Host Immune Response

27:03 Dysbiosis Triggers

30:42 Microbiome and Preterm Birth

33:06 Labor Inflammation Pathways

38:10 Pregnancy Microbiome Shifts

39:11 Screening and Group B Strep

40:57 Microbiome Screening Potential

42:06 Global Data Predictive Signatures

43:23 Limits of Current Risk Tools

45:30 Point of Care Profiling Vision

46:50 Swab Mass Spec Barcoding

48:27 Composition Versus Host Response

49:58 Metabolic Dark Matter

52:13 Beyond Bacteria, Viruses, and Fungi

55:48 Vaginal Probiotic Trials

57:39 Phase Two Trial Endpoints

01:01:28 Vaginal Microbiome Transplants

01:05:46 Birth Seeding After C Section

01:10:57 Do Vaginal Strains Persist

01:14:05 Ten-Year Personalised Medicine

01:17:45 Wrap Up And Where To Find