40 y/o female with 4 children complains of amenorrhea for 3 months. States that she thinks this is early menopause, if she is pregnant she will be “very upset.” Based on this scan, will she be 1x, 2x or 3x “very upset?”
Answer: Monoamniotic triplets, 3x “very upset”
This scan demonstrates three intrauterine fetuses sharing the same gestational sac (mono-amniotic), early second trimester. There is no evidence of free fluid in the pelvis, and all three fetuses have identifiable heart beats.
Monozygotic triplets occur at a rate of 22.6 to 82.9 per million births, and monochorionic triplets are the least common.1
Although clearly the patient is unlucky with these odds, you can tell your patient that she was far more likely to get struck by lightning (1:12K) or die in a shark attack (1:18K).2,3
Imaizumi Y. A comparative study of zygotic twinning and triplet rates in eight countries, 1972-1999. J Biosoc Sci. 2003;35(2):287-302.