Answers

Short answers

1 The pathological specimen is called a fetus compressus, fetus papyraceus, or "vanishing twin."

2 Complications in the mother could be disseminated intravascular coagulation, obstructed labour, or preterm delivery.

Possible complications in the fetus include cerebral palsy, fetal distress during delivery, intrauterine growth retardation, prematurity, aplasia cutis, and congenital anomalies such as gastroschisis, absence of ears, and intestinal atresia.

3 Differential diagnoses include placental cysts; placental tumours such as chorioangioma, which are usually more vascular than the mummifying fetus; subchorionic haemorrhage; or yolk sac or extraembryonic coelom, which can be confused for a second fetus on the scan.

Long answers
1. Vanishing twin syndrome—this syndrome occurs in multiple pregnancies when one or more of the fetuses dies, leaving behind a viable one. It was first described by Stoeckel in 1945.¹ The incidence of vanishing twin syndrome has risen with the increase in multiple pregnancies from in vitro fertilisation techniques. Also, the advent of and advance of ultrasonography mean the loss of a fetus in a multiple gestation is often identified. The reported incidence of vanishing twin syndrome is 1:12 000 live births.²

The prognosis for and growth of the viable fetus is usually excellent. No active interventions are needed, and the condition can be managed conservatively. In most cases death of a fetus occurs in the second trimester, between the third and sixth month. Nearly all deaths occur before the seventh month, leaving enough time for the fetus to mummifiy.

Vanishing twin syndrome occurs in 20-30% of multifetal gestations.³ Before the 10th week of gestation the vanishing rate can be as high as 71%.⁴ Some studies indicate that advanced maternal age is associated with increased incidence of vanishing twin syndrome in multiple gestations.^{5 6} This could be because of the increasing frequency of placental and chromosomal abnormalities in older mothers.

In a multiple gestation the death of the abnormal fetus will usually occur because of chromosomal abnormalities in early development, but fetal death can sometimes be caused by improper cord implantation.⁷ In the dead twin the amniotic fluid and placental contents are resorbed gradually. The growing twin exerts pressure on the dead twin and flattens it.⁷ The shape and size of the dead twin depends on the size of the mother’s uterus and the position of the dead fetus within it, the position of the placenta, the placement of the live twin, and when the fetus died. If the dead twin is compressed it is called a fetus compressus; if it is thin and parched like paper it is called a fetus papyraceus. Sometimes it is resorbed completely and the remnants might be seen on the placenta as amorphous material or nodular calcifications. Using microscopy the placental tissue may show deposits of lamellar fibrin with nests of epithelioid cells with adjacent chorionic villi. ^{8 9}

2 Complications in the mother and the viable fetus—when the vanishing twin dies is important because the timing determines the possible complications for the viable fetus and mother. Usually, if the death occurs in the first trimester no untoward effect on the mother is seen. In some cases, the mother may have mild bleeding and uterine cramping.¹⁰ Death of a fetus during the second or third trimester may cause maternal complications such as preterm labour, consumptive coagulopathy, severe puerperal haemorrhage, or obstructed labour because of a low lying fetus papyraceus.

If the vanishing twin dies in the second and third trimester the effect on the viable fetus includes an increased risk of cerebral palsy.^{7 11} Cerebral palsy in the surviving twin may occur because reversed blood flow from the macerated twin to the viable twin allows an increase of thromboplastins into the circulation of the live fetus, causing disseminated intravascular coagulation. This can lead to damage to the intrauterine central nervous system and various other congenital anomalies such as gastroschisis, intestinal atresia, and, rarely, ears being missing.⁷ Other forms of morbidity in the surviving twin also include prematurity, intrauterine growth retardation, aplasia cutis, or areas of skin necrosis. ⁷

3 Differential diagnosis^{9 12}—a mother with vanishing twin syndrome should have routine antenatal checks. Her levels of β human chorionic gonadotrophin will rise more slowly than in a mother with a normal twin pregnancy. She must have ultrasonography with regular follow-ups throughout the pregnancy to confirm multifetal gestation, the loss of the fetus, and its gradual disappearance during the pregnancy. A mother with this syndrome should be managed conservatively. The pregnancy is classified as high risk. Complications such as disseminated intravascular coagulation or infections, if they arise, should be managed appropriately.

The viable twin does not require any special care unless there is evidence of fetal distress, in which case active interventions may be needed. After delivery, examining the placenta and baby for any deformities is important. Obtaining histological samples of the placenta is essential because it might be the only evidence that the dead fetus existed in a suspected case of vanishing twin syndrome. ¹²

Cite this as: BMJ 2008;337:a2453