Definitions

1. Vanishing Twin Syndrome (VTS)

A condition in which a multiple gestation is identified, but one or more embryos or fetuses later dies and is no longer visible on ultrasound; tissue may be resorbed completely, partially, or compressed as a fetus papyraceous.

2. Fetus Papyraceous

A fetus that dies in the second or third trimester and becomes flattened and mummified due to pressure from the surviving fetus.

3. Blighted Ovum

A blighted ovum occurs when a gestational sac develops, but the embryo inside stops developing very early and is subsequently resorbed, leaving an empty-appearing sac visible on ultrasound.

4. Miscarriage

The spontaneous loss of a pregnancy before 20 weeks’ gestation in the United States (or before 22 weeks in many international systems). It includes both embryonic loss and early fetal loss.

5. Early Miscarriage or Early Pregnancy Loss

Loss of a pregnancy before 13 weeks’ gestation.

6. Late Fetal Loss

The spontaneous death of a fetus between 10–20 weeks’ gestation in some systems, or between 13–22 weeks in others, depending on national definitions. It represents the stage between early miscarriage and intrauterine fetal demise, when the fetus is developed enough to be recognizable but has not yet reached the gestational age threshold that defines a stillbirth.

7. Intrauterine Fetal Demise (IUFD)

Refers to the death of a fetus in utero prior to delivery, regardless of gestational age. The term describes a clinical event rather than a specific legal or statistical classification.  In practice, how IUFD is categorized for reporting purposes depends on gestational age, fetal weight, and jurisdiction:  

• In the United States, New Zealand, and Australia, as well as other nations, fetal death occurring at ≥20 weeks gestation is typically reported as a fetal death or stillbirth (or ≥350-400 grams when age is uncertain, depending on jurisdiction).

• Some regions (e.g., the UK, Netherlands, and others) use ≥ 24 weeks gestation as the threshold to classify fetal death as stillbirth. 

• The World Health Organization (WHO) and nations (e.g., China, India, and others), define stillbirth as fetal death occurring at ≥28 weeks gestation.

Losses before these thresholds are generally classified as miscarriage, early pregnancy loss, or late fetal loss, depending on timing and system used. The term IUFD is commonly used in clinical documentation across gestational ages, even when the loss does not meet the legal definition of stillbirth in a given jurisdiction (e.g., with losses diagnosed as IUFD before 20–24 weeks). Even within a given jurisdiction, application of the term and its thresholds can vary. See our Memorialization/Disposal of Remains page to learn more.

8. Anembryonic Pregnancy

An anembryonic pregnancy is a broad, umbrella term under which a blighted ovum would belong; it is a clinical term describing any pregnancy in which no embryo is identified. 

9. Missed Abortion / Missed Miscarriage

A pregnancy loss where the embryo or fetus has died but remains in the uterus without expulsion. (e.g., fetus papyraceous or an embryo that did not full resorb)

10. Embryonic Demise

The death of an embryo before 10 weeks’ gestation, typically identified on ultrasound when an embryo is visible but shows no cardiac activity or has stopped growing compared to expected developmental milestones.

11. Pregnancy of Unknown Location (PUL)

Positive pregnancy test with no identifiable intrauterine or ectopic pregnancy on ultrasound.

12. Multiple Gestation

A pregnancy involving more than one embryo or fetus (e.g., twins, triplets, etc.).

13. Surviving Twin / Surviving Multiple

The primary term utilized in medical literature to denote the fetus, infant, child, or adult who continues to live after loss of a co-twin or co-multiple at any point in life.

14. Womb Twin

This term is not widely recognized in the medical guidelines, and is primarily seen in psychology and bereavement literature where it describes individuals who survive a prenatal loss of multiples, similar to surviving twin/multiple.

15. Chorionicity

The number of placentas in a multiple pregnancy (dichorionic = two placentas; monochorionic = one shared placenta). Note: Chorionicity (DCDA, MCDA, MCMA) cannot determine zygosity on its own; genetic testing is required for confirmation.

16. Amnionicity

The number of amniotic sacs (diamniotic = two sacs; monoamniotic = one shared sac).

17. Zygosity

Zygosity refers to the genetic relationship between fetuses in a multiple pregnancy and describes whether they originate from one fertilized egg (monozygotic) or two separate fertilized eggs (dizygotic).

• Monozygotic (identical) twins arise from the division of a single fertilized egg and have the same genetic material.

• Dizygotic (fraternal) twins arise from two separate eggs fertilized by two different sperm and are genetically similar to typical siblings.

Zygosity is determined at conception and remains fixed throughout pregnancy. It is distinct from chorionicity and amnionicity, which describe placental and sac structures and depend on the timing of embryonic division.

Note: Zygosity cannot always be determined by ultrasound findings alone and may require genetic testing for confirmation.

18. Dizygotic Twins

Twins from two separate eggs fertilized by two separate sperm (fraternal twins). Usually dichorionic diamniotic (DCDA). However, DCDA twins can be either identical OR fraternal. Identical twins can be DCDA if the split happens very early. All dizygotic twins are DCDA, but not all DCDA twins are dizygotic. 

19. Monozygotic Twins

Twins resulting from the division of a single fertilized egg (identical twins).

One fertilized egg splits; placentation and amnionicity depend on timing:

a. DCDA (division at 0-3 days gestation)

• Two placentas, two separate sacs

b. MCDA (Monochorionic–Diamniotic) (division at 3-8 days gestation)

• One shared placenta, two separate sacs

c. MCMA (Monochorionic–Monoamniotic) (division at 8-13 days gestation)

• One shared placenta, one sac

d. Conjoined Twins (incomplete division at 13+ days gestation)

• Physical connection due to incomplete embryonic separation

• Twins typically develop recognizable anatomy and may share organs or structures

20. Cell-Free DNA (cfDNA)

Placental DNA fragments circulating in maternal blood used for noninvasive prenatal screening.

21. Maternal Serum Screening (MSS)

Blood tests evaluating biomarker levels to estimate risk for fetal abnormalities.

22. Chimerism

Presence of two genetically distinct cell lines in one individual.

23. Mosaicism

Presence of multiple genetically distinct cell populations originating from a single zygote.

24. Nuchal Translucency (NT)

Fluid space measurement at the fetal neck during first-trimester ultrasound used in screening.

25. Crown-Rump Length (CRL)

Measurement from the fetal head to the rump used for accurate pregnancy dating.

26. Placental Pathology

Examination of placental tissue to identify abnormalities or disease.

27. Intrauterine Growth Restriction (IUGR)

A condition where one or more fetuses do not reach expected growth potential. 

28. Selective Fetal Growth Restriction (sFGR)

A complication in multiple pregnancies where one fetus grows significantly slower than the co-twin(s).

29. Preterm Birth

Birth occurring before 37 completed weeks of gestation.

30. Twin-to-Twin Transfusion Syndrome (TTTS)

Acute hemodynamic imbalance with fluid discordance.

31. Twin Anemia–Polycythemia Sequence (TAPS)

A chronic, slow blood imbalance between monochorionic twins that leads to anemia and polycythemia (with little to no fluid discordance). 

32. Perinatal Grief

Emotional and psychological grief associated with pregnancy or infant loss.

33. Bereavement Care

Support services for individuals and families experiencing pregnancy or neonatal loss.

34. Peer Support

Peer support is a form of help in which people with shared lived experiences provide emotional, informational, and practical support to one another. It is grounded in mutual understanding rather than professional authority.

35. Peer Review 

Peer review is a formal evaluation process in which experts in a specific field assess the quality, accuracy, rigor, and significance of a researcher’s work before it is published or funded.

36. Research Translation

Research translation is the process of taking scientific findings and transforming them into practical applications, policies, programs, clinical guidelines, educational tools, and real-world improvements. It bridges the gap between research evidence and practice by making complex scientific knowledge understandable, accessible, and actionable for non-research audiences.

37. Mirror-Image Twins

Mirror-image twinning refers to a subset of monozygotic twins in which lateralized anatomical, morphological, or functional traits develop in reversed orientation between co-twins, arising from delayed embryonic splitting after the establishment of the left–right body axis.

38. Parasitic Twin

A type of asymmetric conjoined twinning in which one twin is severely underdeveloped and non-viable, depending on the host twin for survival. The parasitic twin lacks essential organs and is supported entirely by the host’s circulatory system, usually presenting as an externally attached underdeveloped mass.

39. Fetus in Fetu (FIF)

A rare condition in which a malformed embryonic structure becomes encased within its co-twin during early development. FIF is characteristically found inside the host twin’s body, often with partial vertebral or limb-like organization, distinguishing it from a teratoma or external parasitic twin.

40. Teratoma

A type of germ cell tumor composed of tissues from two or more embryonic germ layers (i.e., ectoderm, mesoderm, and/or endoderm). Because germ cells are pluripotent, teratomas can contain a wide variety of tissue types (e.g., hair, teeth, bone, muscle, thyroid tissue, or neural tissue). Fetus in fetu (FIF) differs from a teratoma because FIF contains an organized, embryo-like body structure, while a teratoma is an unorganized tumor made of mixed germ-cell tissues.

41. Hydatidiform Mole

A hydatidiform mole is an abnormal gestation characterized by pathologic trophoblastic proliferation and hydropic degeneration of chorionic villi. Partial hydatidiform moles are typically triploid, whereas complete moles are diploid and androgenetic. Triploidies and hydatidiform moles can overlap only in partial molar gestations.

Complete Mole:

• Genotype usually 46,XX or 46,XY, entirely paternal in origin (androgenetic), arising from fertilization of an enucleated ovum.

• No embryo or fetal tissue develops.

Partial Mole:

• Genotype is typically triploid (69,XXX; 69,XXY; 69,XYY).

• Occurs when a normal ovum is fertilized by two sperm (dispermy) or, rarely, a diploid sperm.

• May show some fetal tissue but is nonviable.