Why does the uterus refuse to implant the embryo after implantation it?

If the uterus refuses to sustain an embryo after implantation, it can be due to various underlying factors that affect the stability of pregnancy. This condition is often referred to as recurrent implantation failure (RIF) or early pregnancy loss (chemical pregnancy or miscarriage). Here are some possible reasons:

1. Genetic Abnormalities in the Embryo

• Chromosomal abnormalities (aneuploidy) in the embryo can lead to implantation failure or early miscarriage.
• Abnormal DNA fragmentation in sperm or eggs can affect the embryo’s viability.
• Defective blastocyst formation may prevent proper development.

2. Uterine Abnormalities

• Thin or defective endometrial lining: A uterine lining <7mm is often inadequate for sustaining implantation.
• Fibroids, polyps, or adhesions: These structural issues can interfere with embryo implantation.
• Congenital uterine malformations (e.g., septate uterus): May lead to recurrent pregnancy loss.

3. Hormonal and Immune Dysfunction

• Low progesterone levels: Insufficient progesterone (luteal phase defect) can cause improper endometrial receptivity.
• Thyroid dysfunction: Hypothyroidism or hyperthyroidism can disrupt implantation.
• Elevated prolactin levels: High prolactin (hyperprolactinemia) can impair progesterone function.
• Autoimmune disorders: Conditions like antiphospholipid syndrome (APS) or maternal immune rejection can cause implantation failure.

4. Immunological Factors

• Overactive natural killer (NK) cells: The immune system may attack the implanted embryo.
• HLA incompatibility: If the immune system perceives the embryo as foreign, it may reject it.
• Inflammatory conditions: Endometriosis or chronic endometritis can create an inhospitable uterine environment.

5. Blood Clotting Disorders (Thrombophilia)

• Conditions like Factor V Leiden mutation, Protein C or S deficiency, or Antiphospholipid Syndrome can cause microclots, restricting blood flow to the embryo.

6. Infections

• Chronic infections such as bacterial vaginosis, chronic endometritis, chlamydia, or tuberculosis can lead to implantation failure.

7. Poor Embryo Quality (IVF Cases)

• In assisted reproduction (IVF), poor-quality embryos may fail to thrive despite successful transfer.
• Issues like defective zona pellucida or delayed embryo development can impact survival.

8. Environmental & Lifestyle Factors

• Obesity or being underweight affects hormonal balance.
• Smoking, alcohol, and caffeine consumption can reduce implantation success.
• Excessive stress increases cortisol, which can impact the immune system and endometrial receptivity.

Possible Treatments & Solutions

• Progesterone supplementation for luteal phase support.
• Low-dose aspirin or heparin for clotting disorders.
• Steroids or immunotherapy for immune-related issues.
• Lifestyle changes (healthy weight, diet, reducing stress).
• Endometrial scratching to improve receptivity.
• Preimplantation Genetic Testing (PGT-A) to select healthy embryos during IVF.

If implantation failure occurs multiple times, a fertility specialist should evaluate possible causes through blood tests, ultrasound, genetic screening, and endometrial biopsy.

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