OBJECTIVES: To determine whether chorioamniotic membrane separation from the 
internal cervical os, the "moon sign," is associated with preterm premature 
rupture of membranes (PPROM) in twin-twin transfusion syndrome (TTTS).
METHODS: A retrospective study of patients with TTTS treated with laser surgery 
was performed. Membrane separation before and after surgery was tested against 
any PPROM, PPROM within 7 days, and PPROM within 21 days. Because intrauterine 
fetal demise (IUFD) was weakly associated with PPROM, these cases were studied 
separately.
RESULTS: Among 304 consecutive patients, 247 patients (81.3%) had no IUFD, and 
preoperative and postoperative membrane separation rates were 13.4% and 13.0%, 
respectively. In 7 cases (2.8%), preoperative membrane separation disappeared 
postoperatively, and in 6 cases (2.4%), membrane separation appeared 
postoperatively; 26 cases (10.5%) had membrane separation at both times. Rates 
of PPROM did not differ between those who did and did not have preoperative 
membrane separation (30.3% versus 28.0%; P= .9511). Among those with and without 
postoperative membrane separation, the rates of any PPROM were 34.4% and 27.4%, 
respectively (P = .5473), and the rates of PPROM within 21 days were 15.6% and 
5.6% (P = .0524). Those with postoperative membrane separation were 3 times more 
likely to have PPROM within 21 days (odds ratio, 3.13; 95% confidence interval, 
1.02-9.58; P= .0453). Preterm premature rupture of membranes was not associated 
with preoperative or postoperative membrane separation in patients with IUFD.
CONCLUSIONS: The preoperative moon sign does not appear to be associated with 
PPROM in TTTS. Postoperatively, membrane separation may be weakly associated 
with PPROM at 21 days, but further research is required to confirm this 
association.