Why does Woman’s body Reject Pregnancy?

UPDATED ON 23 MAY 2025

Understanding the Immunological Causes behind Failed Implantation and Pregnancy Loss

At ImmuRepro, we often get asked: “Why does my body reject my pregnancy, even when everything seems right?” It’s an emotional question and a valid one.

As a reproductive immunologist in India, let me walk you through this journey of understanding how and why the immune system, which is supposed to protect us, might sometimes prevent a pregnancy from progressing.

AUTHOR

CONDITION

Repeated Miscarriages

Failed IVF Cycles

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Pregnancy is Not a Disease — It’s a Physiological Marvel

Let me start with something very important: pregnancy is not a disease. It’s a natural extension of the female body. The sophisticated biological event where the body allows the growth of something that is partially foreign to it. This is what makes pregnancy so fascinating.

Think about it. In most cases, our body’s immune system reacts aggressively toward anything foreign, be it a virus, bacteria, or an organ transplant. Yet, when an embryo implants in the uterus — a semi-allergenic graft with 50% genetic material from the father — the immune system chooses tolerance. Why and how does this happen? That’s the core of reproductive immunology.

How Does the Immune System Usually React to Foreign Invaders?

In simple terms:

• When foreign cells like bacteria or viruses enter the body, the immune system recognizes them and attacks.

• The same thing happens in organ transplants — if the donor kidney isn’t matched properly or immunosuppressants aren’t used, the body may reject it.

 Now here’s the amazing part…

Pregnancy: A Natural Immunological Exception

Despite the embryo containing 50% paternal genetic material, the female body doesn’t attack it. In fact, the immune system adapts and sets up what we call an “immunomodulatory response” to support and protect the pregnancy.

This is why we refer to pregnancy as an immunomodulatory state.

What Makes Pregnancy Immunologically Unique?

Pregnancy challenges the immune system to strike a balance — not too reactive to reject the fetus, but responsive enough to fight infections. Here’s how it works:

The Role of Uterine NK Cells (uNK Cells) and HLA-C Antigens

• These special immune cells in the uterus are not the same as regular NK (natural killer) cells in the bloodstream.

• They play a key role in helping the embryo implant and establishing the placenta.

• In immunohistochemistry (IHC), they have a different signature, making them specialized for pregnancy.

Embryo’s Contribution: HLA-C Antigen

• Unlike other cells, the embryo presents HLA-C antigens instead of HLA-A or B.

• This unique antigen presentation prevents a full-scale immune rejection.

This harmony between the embryo’s HLA-C and the mother’s uNK cells allows the embryo to:

• Embryo invasion into spiral arteries

• Placenta formation

• Successful pregnancy growth

What Can Go Wrong: Why the Immune System Rejects a Pregnancy?

Any disruption in this immunological interplay can lead to:

• Failed implantation
• Pregnancy loss

Let’s go over some common causes that can trigger an immune rejection:

1. Pelvic Pathologies

• Fibroids
• Endometriosis
• Adenomyosis

These conditions may alter the local immune response, affecting embryo acceptance.

2. Pelvic Infections

Chronic or acute infections can lead to inflammation and immune activation, causing the uterus to reject the embryo. 

3. Autoimmune Disorders

• Systemic Lupus Erythematosus (SLE)
• Autoimmune thyroiditis
• Rheumatoid arthritis

These disorders result in misguided immune attacks, which can affect embryo survival (pregnancy)

4. Müllerian Anomalies

Congenital anomalies in the uterus can make it less supportive of implantation.

5. Endocrinological Factors

Hormonal imbalances can indirectly impact the immune tolerance mechanisms.

6. Poor Hygiene

Improper genital hygiene may lead to recurrent infections, affecting the local immune response.

7. Chromosomal Abnormalities in the Embryo

If the embryo itself has genetic defects, rejection by the immune system is often nature’s way of preventing an unhealthy pregnancy.

If the embryo is genetically abnormal, immune rejection is protective. But if a normal embryo is lost, we must evaluate further.

When Should You See a Reproductive Immunologist?

We recommend an evaluation:

• After repeated miscarriages
• After a failed IVF cycle

At ImmuRepro, we follow a holistic, ethical, and scientific approach:

• Only 30% of patients require immunological treatment.

70% do not need any medication — just proper evaluation and reassurance.

Why Does Immunological Assessment Matters?

• Understand the root cause of unexplained pregnancy loss
• Avoid unnecessary treatments
• Create a customized plan to support successful pregnancy

Our goal is not to medicate every patient but to support only those who genuinely need help. In a country like India, this non-commercial, science-backed approach is crucial.

Key Takeaways

• Pregnancy is an immunological exception, not a flaw.
• A balance between immune tolerance and protection is essential.
• Disruptions in this system can lead to failed pregnancies.
• Most causes are diagnosable and manageable.

Not all patients require treatment — personalized evaluation is key.

Let’s Evaluate Together

If you’ve faced a miscarriage or failed IVF, you’re not alone. There is hope — but it begins with understanding. Let us help you assess the immunological factors involved and guide you through your fertility journey with care, empathy, and science.

AUTHOR

CONDITION

Repeated Miscarriages

Failed IVF Cycles

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