amniocentesis for prenatal karyotyping is today the most frequently performed invasive intrauterine procedure ( eisenberg and wapner , 2002 ) , but still carries a procedure related risk for amniotic fluid leakage estimated to be approximately 1 - 2% ( reece , 1997 ; tabor et al . , 1986 ) . postprocedural amniotic fluid leakage is the strongest predictor of subsequent fetal loss ( saltvedt and almstrom , 1999 ) . instrument size influences the created defect in the fetal membranes and the subsequent risk for amniotic fluid leak and miscarriage . traditionally , 20 and 22 g spinal needles have been used for amniocentesis , although remarkably , this was not based on experimental data . more recently , a new needle was designed and marketed specifically for invasive intrauterine procedures ( 23 g : cook echotip , spencer , indiana , usa ) . this smaller 23 g needle potentially represents less trauma to the uterus and fetal membranes . additionally , the specifically designed tip ( echotip , fig . 1 ) is designed to enhance ultrasound visualisation and could reduce the sampling time , and therefore the discomfort for the patient and risks of the procedure . the aim of this study was to compare this new needle type with a standard 22 g spinal needle in terms of defect size created in the fetal membranes and time needed to perform an invasive intrauterine procedure . fetal membranes were freshly obtained from four women delivered by elective caesarean section at term ( 37 - 40 weeks ) following uncomplicated pregnancies . membrane patches of 100    50  mm were excised and divided in two equal parts . each membrane patch was wrapped around the bottom of a plastic tube and secured with an elastic ring . the tube was fixed in a vertical position and then filled with hartmann s solution or amniotic fluid at 37c ( fig . the fluid pressure was kept constant at 15  cm h2o representing a physiologic value for amniotic pressure in pregnancy ( fisk et al . , 1992 ) . the membranes were punctured by introduction of the needle at an angle of 90 1 to 1.5  cm deep through the bulging membranes using either a 22 g or 23 g needle . after the experiment , the membrane was detached and the wound edge stained with india ink to facilitate recognition during microscopic evaluation . we calculated the size ( mm ) of the defect using a microscope ( zeiss , oberkauchen , germany ) , at magnification ( 20 ) and adapted software ( ks 100 3.0 , carl zeiss , oberkochen , germany ) . in cases where sliding of the amnion and chorion against each other had resulted in a smaller net defect , this net defect through both the amnion and chorion , was measured . 22 g : vygon , valkenswaard , the netherlands 23 g : cook echotip , spencer , indiana , usa * calculated as the cross - sectional area , based on the outer diameters provided by the manufacturer . as a second part of the study , we determined the performance of six operators with different levels of clinical experience [ novice ( n  =  2 ) , intermediate ( n  =  2 ) , expert ( n  =  2 ) ] , using either a 22 g or 23 g needle for cordocentesis in a randomised video - controlled setting ( silver et al . , 1998 ) . all procedures were performed on a cordocentesis simulator ( limbs & things ltd , bristol , united kingdom ) . this simulation model allows for the combination of realistic ultrasound visualisation as well as mock blood sampling from an anterior or posterior placenta with umbilical cord ( fig . ultrasonography was performed using the same machine we use for all our invasive intrauterine procedures , an acuson sequoia ( mountain view , ca ) with a 2.5 - 6  mhz transducer . each operator performed four cordocentesis procedures ( single operator , two hands technique ) with each needle . in total , 24  procedures were performed with each needle . each operator performed only one procedure per day . for each procedure , we measured the procedure time , defined as the time from puncturing the cordocentesis simulator to successfully collecting 2  ml of artificial blood , in seconds . after completion of the project , measurement of the procedure - times was performed from the videotapes by two co - workers blinded to the type of needle that was used during the experiment . results were stored and analysed using spss 10 ( spss inc . , chicago , illinois ) software and are expressed as mean    sd . the expected and actual defects created by both 22 g and 23 g needles are shown in table 1 . the defect sizes created by 22 g needles were slightly larger than those created by 23 g needles : 0.66+/-0.12 versus 0.59+/-0.13  mm . in accordance to previous observations , the defect size created by both needles was larger than the expected defect size based on the cross - sectional diameter of the needle ( devlieger et al . , 2002 ) . the mean sampling duration for all operators pooled was similar : 144    188  sec with a 22 g needle versus 144    120  sec with a 23 g needle , p  =  0.99 . the mean duration of sampling for the different study groups is shown in table 2 . there were no significant differences in duration of sampling between the two needles in the three study groups , however , 4 of the 6 operators required less time for cordocentesis with the 22 g needle as compared to the 23 g needle . despite the increasing number of pregnant women undergoing invasive intrauterine procedures and the not infrequent , potentially dramatic complications , few experimental studies have focused on the technical aspects of these procedures . as a general rule , to reduce the risks of intrauterine procedures , it seems logical to use the least traumatic needle that combines fast sampling and easy ultrasound visualisation . in today s clinical practice , most fetal medicine specialists use 22 g spinal needles to perform amniocentesis ( eisenberg and wapner , 2002 ) . we have shown that this needle creates a small defect in the fetal membranes reducing subsequent fluid leakage , as compared to bigger needles ( devlieger et al . , 2002 ) . smaller needles with a diameter of 24 g and 26 g are associated with increased sampling time , which results in relatively larger defects , probably as a result of increased manipulation and physiologic tremor ( devlieger et al . , 2002 ) . additionally , these very thin needles can be blocked by particles in amniotic fluid leading to unsuccessful sampling . larger needle diameters are associated with slightly shorter sampling time , but exponentially larger defects in the fetal membranes ( devlieger et al . , 2002 ) . therefore , a 22 g needle appears to provide a good compromise between sampling time and defect size . only recently , needle diameters , shape and ultrasonographic properties the 23 g needle with echo - tip has several theoretical advantages , but these were not confirmed in this experimental study . first , the specially designed echotip was expected to give a better ultrasonographic image than the needles currently used . this should help the operator in determining the location of the needle inside the amniotic sac and lead to a decrease in sampling time . however , most operators did not have shorter sampling times with the 23 g needle , on the contrary : 4 out of 6 operators performed better with the 22 g needle . in general , the oblique tip of a standard needle sufficiently enhances sonographic visualisation . the additional reflections created by the 23 g echotip not only seem unnecessary , but may even have a negative effect on precisely localizing the needle tip . a possible limitation of our study could be the use of both an excellent ultrasound machine and a good model ; we can not exclude that the echotip could be of more value with the use of less advanced equipment or in procedures done in obese patients . secondly , the defect created by the 23 g needle was not significantly smaller than the fetal membrane defect created with a 22 g needle . theoretically , the cross - sectional area of the needle determines the minimal size of the created defect . this cross - sectional area relates to the needle diameter according to the equation r . therefore , the relationship between needle diameter and defect size can be expected to be exponential ( fig . defects created by both 22 g and 23 g needles are in the horizontal part of this exponential curve ( devlieger et al . , 2002 ) , explaining the small differences measured . of course , the relatively small sample size used in this study could also contribute to the fact that no significant difference was noted in our experiments . using the cordocentesis model also , procedures performed on the model are not complicated by a moving fetus and complications like prom do not occur . nonetheless , with the blinded randomised design , we would not expect the results of our study to be different if performed in the actual clinical setting . in summary , a 23 g needle with adapted tip for enhanced ultrasound visualisation did not , in comparison with a 22 g spinal needle , result in decreased defect size in the fetal membranes or decreased sampling time in a randomised experimental setting . therefore , the claimed advantages of the more expensive 23 g needle over the 22 g needle are not supported by our study .