Spectrophotometry of amniotic fluid: a simple and rapid method for distinguishing between gestational sacs in second-trimester amniocentesis of twin pregnancies.

Objective: To evaluate the use of an alternative and simple spectrophotometric method that could replace the intraamniotic injection of dye in the second-trimester amniocentesis of twins.

Methods: Optical densities (OD) of amniotic fluid (AF) obtained at second-trimester genetic amniocentesis were measured at 360 nm. Comparison was made between 36 twin pregnancies and 60 singleton controls.

Results: The mean difference in OD between two samples taken from the same sac of singleton pregnancies was very small (mean 0.004, standard deviation [SD] 0.003). However, samples drawn from each of the twin pairs' amniotic sacs varied widely (raw mean 0.176, SD 0.223; median 0.096, range 0.011-0.943). The difference between the groups was highly significant statistically (P < .001).

Conclusion: Our preliminary experience with spectrophotometry of AF in twin amniocentesis suggests that this simple test may be useful in ascertaining that the AF samples were obtained from each of the two gestational sacs.