Implantation Disorders and Recurrent Pregnancy Loss

Implantation disorders and recurrent pregnancy loss cover the failure of an embryo to establish or sustain a viable pregnancy, ranging from repeated failure of transferred embryos to implant to the recurrence of clinical miscarriage. This area organizes the definitions, established and contested causes, and diagnostic approaches that frame how repeated reproductive loss is studied and classified.

Definition

Recurrent pregnancy loss is conventionally defined as two or more failed clinical pregnancies, and implantation failure as the repeated failure of good-quality embryos to implant; together they describe the spectrum of repeated reproductive loss before viability.

Scope

The area spans the definition and epidemiology of recurrent pregnancy loss, the chromosomal and genetic causes of miscarriage, the biology and assessment of endometrial receptivity and implantation, and the haematological and immunological factors investigated as contributors to repeated loss. It is a reference orientation to a heterogeneous clinical field and does not provide management protocols.

Sub-topics

• Chromosomal Abnormalities and Aneuploidy as Causes of Miscarriage
• Immune Factors and Natural Killer Cells in Recurrent Loss
• Implantation Failure and Receptivity Assessment
• Recurrent Pregnancy Loss: Definition, Epidemiology and Classification
• Thrombophilia, Antiphospholipid Syndrome and Recurrent Loss

Core questions

• How are recurrent pregnancy loss and implantation failure defined, and why do thresholds differ between guidelines?
• Which causes of repeated loss are established versus contested?
• How is endometrial receptivity assessed, and how strong is the supporting evidence?
• What proportion of recurrent loss remains unexplained after investigation?

Key concepts

• Recurrent pregnancy loss
• Recurrent implantation failure
• Embryonic aneuploidy
• Endometrial receptivity and the window of implantation
• Antiphospholipid syndrome
• Unexplained recurrent loss

Mechanisms

Repeated loss can arise at distinct points: before implantation, when a chromosomally abnormal or developmentally impaired embryo fails to attach; at implantation, when the endometrium is non-receptive or the maternal-fetal interface is dysfunctional; and after implantation, when established pregnancies are lost. Embryonic aneuploidy accounts for a large share of sporadic and recurrent early loss, while a minority of cases are attributable to parental structural chromosome rearrangements, antiphospholipid syndrome, uterine anatomical factors, or endocrine disturbance. A substantial proportion remains unexplained after standard evaluation.

Clinical relevance

Repeated pregnancy loss and implantation failure are major reasons couples seek reproductive evaluation, and the distinction between established and unproven causes shapes how evidence in this field is interpreted. This entry is a reference orientation to the topic and describes how the field is structured; it is not a basis for individual diagnosis or treatment.

Epidemiology

Sporadic miscarriage affects a large fraction of recognized pregnancies, whereas recurrent pregnancy loss, defined as two or more losses, affects a smaller proportion of couples trying to conceive. Risk rises with maternal age, largely reflecting the increasing frequency of embryonic aneuploidy.

Evidence & guidelines

Major society guidelines, including those of ESHRE and the American Society for Reproductive Medicine, define recurrent pregnancy loss and outline recommended investigations, while emphasizing that many proposed causes and interventions rest on limited or conflicting evidence.

History

Recurrent miscarriage was long described as habitual abortion and approached empirically. Over recent decades, cytogenetic analysis of pregnancy tissue, the recognition of antiphospholipid syndrome, and advances in reproductive medicine reframed the field around defined causes, evidence-based investigation, and a candid acknowledgement that much loss remains unexplained.

Debates

Should recurrent pregnancy loss be defined as two losses or three?

Guidelines differ on the number of losses required for the label, which affects who is investigated and how epidemiological figures are reported.

Key figures

• Lesley Regan
• Raj Rai
• Mary Stephenson
• Siobhan Quenby

Seminal works

• rai-regan-2006
• eshre-rpl-2018
• asrm-definitions-2013

Frequently asked questions

What is the difference between recurrent pregnancy loss and recurrent implantation failure?

Recurrent pregnancy loss refers to repeated clinical miscarriage of established pregnancies, whereas recurrent implantation failure refers to repeated failure of transferred embryos to implant; they overlap but describe loss at different stages.

Is a cause always found in recurrent pregnancy loss?

No. Even after a thorough evaluation, a substantial proportion of recurrent pregnancy loss remains unexplained, which is one of the defining challenges of the field.