Superfecundation and superfetation: lessons from the past on early human development

Tubal superfetation following frozen-thawed single-embryo transfers in two separate cycles: Case report and literature review

Superfetation is a very rare occurrence. In the context of assisted reproduction, it has been reported only as an intrauterine pregnancy after ovarian stimulation and/or embryo transfer in the presence of an undiagnosed ectopic pregnancy. Here we report a case of a 27-year-old anovulatory patient, gravida 1 para 1, who underwent two frozen-thawed single-blastocyst transfers in separate cycles. The patient reported that 12 days after the first transfer, she had menstrual bleeding and stopped her estradiol and progesterone supplementation without undergoing a blood human chorionic gonadotropin (βhCG) test. At her request, a second cycle was immediately initiated, with endometrial thickness measuring 4 mm. Eleven days after the second transfer, the βhCG value was inappropriately high. A right tubal pregnancy corresponding to 8 gestational weeks was diagnosed. Laparoscopy revealed a prominent right tubal pregnancy in addition to a significantly smaller left tubal pregnancy. The discordant tubal pregnancies were confirmed histologically. To our knowledge, superfetation involving a second ectopic pregnancy coexistent with a first, contralateral ectopic pregnancy consequent to consecutive in vitro fertilization procedures has not previously been described in the medical literature. This case emphasizes the importance of routine βhCG testing after every IVF cycle, even if apparently unsuccessful.

Heteropaternal Siblings Misclassified as Dizygotic Twins: A Potential Biasing Factor for Heritability Estimates?

Heteropaternal superfecundation (HP) occurs when two or more ova are fertilized by sperm from separate males. The resulting siblings are genetically equivalent to half-siblings and share, on average, 25% of their inherited genetic material. In the absence of genetic testing HP siblings could be treated as dizygotic (DZ) twins in behavioral genetic analyses and bias heritability estimates in phenotypic decomposition models. However, the extent to which such misclassification could affect calculated estimates of heritability is currently unknown. Employing simulation analyses, the current study assessed the potential biasing impact across a variety of conditions varying by proportions of DZ twins, sample sizes, and low, moderate, and high levels of genetic and environmental contribution to phenotypic variance. Overall, the results indicated that misclassified HP siblings had minimal impact on estimates of heritability. Nonetheless, greater attention should be paid to the identification of HP siblings within existing and future twin datasets.

A review of the mechanisms and evidence for typical and atypical twinning

The mechanisms responsible for twinning and disorders of twin gestations have been the subject of considerable interest by physicians and scientists, and cases of atypical twinning have called for a reexamination of the fundamental theories invoked to explain twin gestations. This article presents a review of the literature focusing on twinning and atypical twinning with an emphasis on the phenomena of chimeric twins, phenotypically discordant monozygotic twins, mirror-image twins, polar body twins, complete hydatidiform mole with a coexistent twin, vanishing twins, fetus papyraceus, fetus in fetu, superfetation, and superfecundation. The traditional models attributing monozygotic twinning to a fission event, and more recent models describing monozygotic twinning as a fusion event, are critically reviewed. Ethical restrictions on scientific experimentation with human embryos and the rarity of cases of atypical twinning have limited opportunities to elucidate the exact mechanisms by which these phenomena occur. Refinements in the modeling of early embryonic development in twin pregnancies may have significant clinical implications. The article includes a series of figures to illustrate the phenomena described.

Case report: a black and white twin

Albinism is an autosomal recessive disorder that is caused by a defective synthesis of melanin, resulting in a generalized reduction of pigmentation in the skin, hair and eyes, and leading to an increased risk of skin cancer and vision problems. We report a case of a 22-year-old primigravida of Negroid origin who delivered dichorial diamniotic twins: two daughters were born with a totally different appearance. The first child had a light brown skin, black curly hair and brown eyes, whereas the second had a striking white skin, red-blond curly hair and blue eyes. Oculocutaneous albinism (OCA) and heteropaternal superfecundation were considered in the differential diagnosis. Genetic testing confirmed the diagnosis of OCA type 2 in the second child. The diagnosis of albinism has clinical implications and must be considered when a black and white twin is born.