Timing of early amniocentesis as a function of membrane fusion

PURPOSE: The purpose of this prospective study was to evaluate sonographically the timing of membrane fusion and to determine its possible effect on the timing of amniocentesis. METHODS: Between May 18, 1998, and January 31, 2002, the status of amnion fusion in pregnant patients at 9-15 weeks' menstrual age was identified in women who were to undergo obstetric sonography. Amniocentesis was performed if even a small area of fused membranes that could be traversed was identified; if the membranes were completely unfused, amniocentesis was delayed. The effect of membrane fusion in terms of the need to reschedule amniocentesis was evaluated. RESULTS: We examined a total of 594 patients. Membrane fusion occurred progressively with increasing menstrual age. One hundred six early amniocenteses were scheduled, and 70 were performed; the others were delayed because the membranes were unfused. Our requirement that an area of membrane fusion be found before we would perform amniocentesis resulted in rescheduling the procedure 24-38% of the time. CONCLUSIONS: Membrane fusion, as seen sonographically, is a function of menstrual age. Even by 15 weeks, a portion of the amnion may be unfused with the chorion. Amniocenteses scheduled for early in the pregnancy may need to be delayed until later, when the membranes are at least partially fused, allowing safe passage of a needle. Delaying the procedure may incur higher expense but may be important in terms of lessening the risk involved.