Vanishing Twin Syndrome

Vanishing twin syndrome is the loss of one twin from a multiple pregnancy in the first trimester, with continuation of the other. The cause is usually a chromosomal abnormality or poor implantation of the lost twin. It matters because the surviving twin usually does well, but the loss can cause bleeding, confusing blood tests and, in later losses of a monochorionic twin, injury to the survivor.

• Risk factors: assisted conception, advanced maternal age, monochorionic placentation, bleeding in early pregnancy.
• Most are lost before 12 weeks and reabsorbed.
• A later loss in the second trimester is managed as a single fetal death in a twin pregnancy.

Women may have light vaginal bleeding, mild cramps and a fall in pregnancy symptoms, or have no symptoms at all and learn of the loss at the next scan. The uterus is smaller than expected for a twin pregnancy.

• Bleeding is often light.
• Closed cervical os.
• Differential diagnosis: threatened miscarriage, subchorionic haematoma, ectopic pregnancy, heterotopic pregnancy, molar pregnancy, wrong dates.
• Complications: bleeding, anxiety and grief, discordant results of screening blood tests, low birth weight and preterm birth in the survivor, rarely injury to the survivor in later loss of a monochorionic twin.

The approach is ultrasound, with documentation of chorionicity before the loss.

• Transvaginal ultrasound: second sac or embryo with no heartbeat, a collapsed sac, or a fetus papyraceus later; confirm a live intrauterine twin with a heartbeat.
• Record chorionicity from the first scan.
• Haemoglobin, blood group and Rhesus status; serum human chorionic gonadotrophin only if the pregnancy location is uncertain.
• Cell free DNA screening may be falsely abnormal because of the lost twin and needs invasive testing to confirm.

The principle is reassurance and surveillance of the living twin.

• Supportive: explain that the loss does not harm the other twin in most; counselling for grief; avoid intercourse while bleeding; teach to attend for heavy bleeding or fever.
• Pharmacological: paracetamol 1 g oral 6-hourly as needed (maximum 4 g in 24 hours); folic acid 400 micrograms oral daily; micronised progesterone 200 mg vaginal at night to 12 weeks for bleeding with a history of miscarriage; oxytocin 10 IU IM at birth.
• Procedures: ultrasound at 12 weeks, 20 weeks and every 4 weeks from 24 weeks for growth; fetal brain MRI at 4 weeks after any death of a monochorionic twin after 14 weeks; delivery on obstetric grounds at 39 to 40 weeks.
• Special groups: RhD negative women give anti-D immunoglobulin 250 IU IM within 72 hours of bleeding before 12 weeks, and 500 IU after 12 weeks.
• Red flags: heavy bleeding, fever, severe pain, reduced movements of the survivor after 20 weeks, abnormal growth.
• Follow-up: scan at 20 weeks for survivor anatomy; growth scans every 4 weeks from 24 weeks; placenta sent for histology after birth.