transvaginal ultrasound - guided oocyte pick - up ( opu ) for assisted reproductive technology ( art ) treatment was first reported in 1985 ( 1 ) . since then , this technique has been rapidly widespread throughout the world , as a replacement for laparoscopic oocyte retrieval . currently , the ovarian stimulation protocol for art treatment is changing from hyperstimulation to mild or minimal stimulation protocol by minimizing the dose of parenteral follicle stimulation or shifting to oral medicine ( 2 ) . as a result , the number of follicles punctured and oocytes retrieved during opu has decreased . in our clinic , the average number of retrieved oocytes among patients aged 3740 years has been reported as 3.7 ( 3 ) , and no anesthesia ( either local or general ) is used during opu in more than half of our patients . however , even when only a single follicle is punctured , oocyte retrieval can be painful . therefore a method for reducing pain during oocyte retrieval is needed , particularly for cases where anesthesia is not used . recently , a newly designed needle for opu was launched with the aim of reducing pain during opu ( 4 ) . the tip of the needle ( i.e. the last 50 mm , the part that penetrates into the tissues ) is thinner than standard , with the remainder of the needle having a larger diameter . an initial study showed that overall pain experienced during opu with the reduced needle was significantly lower than that with the standard needle ( 4 ) . however , there were limitations with that study : several follicles were aspirated during each procedure , and local anesthesia was administrated . for studies investigating the effect of a new needle , it is important to ensure that number of aspirated follicles is the same in both study groups and that perceived pain is not masked by analgesic treatment . therefore , in the present study , an attempt was made to evaluate the effect of reduced needle on pain and vaginal bleeding associated with single - follicle opu in patients who did not receive any analgesic treatment . from may through november 2013 , 100 patients were screened for the study and 75 were enrolled ( figure 1 ) . twenty - five screened patients could not be enrolled in this study because they ovulated before planned oocyte aspiration . therefore , there were no protocol violations and all analyses were performed according to the intention to treat . ovarian follicles were matured by either a natural cycle or a clomiphene citrate cycle . signed informed consent was obtained from all patients , and this study was approved by the institutional board of the sugiyama clinic . flow chart showing patients progression through the study the study was registered with the umin clinical trials registry , study number umin000014 800 . patients were randomized on the day of oocyte retrieval into either the reduced needle ( rn ) group or the standard needle ( sn ) group , with a 1:1 allocation ratio . the needle used for the procedure was not known to the patient but for practical reasons it was known to the treating physician . all opu procedures were performed trans - vaginally , by a single physician , with ultra - sonography to guide the needle . patients were randomly assigned to one of the two groups on the day of oocyte aspiration . in each case , a single follicle with a mean diameter of > 17 mm was aspirated , using either a reduced needle with a 17 gauge body and a 20 gauge tip ( sense , vitrolife , sweden ; rn group ) or a standard 19 gauge needle ( uniever oocyte aspiration needle , japan ; sn group ) . all needles were connected to a syringe ( via a luer connection ) for manual suction , meaning that the aspiration pressure could be controlled as needed . if an oocyte could not be retrieved following puncture , follicle flushing was performed up to 5 times . pain was rated by each patient immediately after the opu by means of a visual analogue scale ( vas ) , comprising a vertical line from 0 cm ( no pain ) to 10 cm ( unbearable pain ) ( 5 ) . all patients were asked by the same medical doctor to mark the vas with their rating of the overall pain for the whole procedure . the pain perceived by patients 30 min after the procedure , captured in the same way as overall pain , was a secondary endpoint . bleeding from the vaginal wall ( at the point of needle penetration ) was also evaluated after opu . bleeding  was defined as continuous signs of bleeding even after the application of direct compression for 30 s , and  no bleeding  defined as cessation of blood loss after direct compression . the in vitro fertilization ( ivf ) procedure used in the present study has been described previously ( 6 ) . the retrieved oocyte was fertilized by conventional insemination or intracytoplasmic sperm injection ( icsi ) three to five hours after follicle aspiration . the presence of two pronuclei with extrusion of the second polar body was taken as the evidence of successful fertilization . embryos were evaluated microscopically before transfer , and those with more than seven blastomeres and less than 10% fragmentation were classified as morphologically good - quality embryos ( mges ) ( 7 ) . embryos were placed transcervically into the uterus using a soft et catheter ( kitazato , japan ) ( 8) . pregnancy was recognized when the development of a gestational sac was evident from transvaginal ultrasound on the 21st day after embryo transfer . the level of pain associated with the procedures is higher when it is performed without anesthesia , and the average vas score for the primary endpoint  overall pain experience  was anticipated to be 5.0 cm with the standard needle . the reduced needle was expected to reduce the vas pain score by 1.2 cm ( 24% ) , a clinically significant difference . forty - four patients would be needed in each study group to demonstrate this difference with 80% power and a significance level of 0.05 ( two - tailed tests ) . fisher s exact test was used to analyze dichotomous data and student s t - test and wilcoxon test when normality distribution was rejected were used for continuous variables from may through november 2013 , 100 patients were screened for the study and 75 were enrolled ( figure 1 ) . twenty - five screened patients could not be enrolled in this study because they ovulated before planned oocyte aspiration . therefore , there were no protocol violations and all analyses were performed according to the intention to treat . ovarian follicles were matured by either a natural cycle or a clomiphene citrate cycle . signed informed consent was obtained from all patients , and this study was approved by the institutional board of the sugiyama clinic . flow chart showing patients progression through the study the study was registered with the umin clinical trials registry , study number umin000014 800 . patients were randomized on the day of oocyte retrieval into either the reduced needle ( rn ) group or the standard needle ( sn ) group , with a 1:1 allocation ratio . the needle used for the procedure was not known to the patient but for practical reasons it was known to the treating physician . all opu procedures were performed trans - vaginally , by a single physician , with ultra - sonography to guide the needle . patients were randomly assigned to one of the two groups on the day of oocyte aspiration . in each case , a single follicle with a mean diameter of > 17 mm was aspirated , using either a reduced needle with a 17 gauge body and a 20 gauge tip ( sense , vitrolife , sweden ; rn group ) or a standard 19 gauge needle ( uniever oocyte aspiration needle , japan ; sn group ) . all needles were connected to a syringe ( via a luer connection ) for manual suction , meaning that the aspiration pressure could be controlled as needed . if an oocyte could not be retrieved following puncture , follicle flushing was performed up to 5 times . from may through november 2013 , 100 patients were screened for the study and 75 were enrolled ( figure 1 ) . twenty - five screened patients could not be enrolled in this study because they ovulated before planned oocyte aspiration . therefore , there were no protocol violations and all analyses were performed according to the intention to treat . ovarian follicles were matured by either a natural cycle or a clomiphene citrate cycle . signed informed consent was obtained from all patients , and this study was approved by the institutional board of the sugiyama clinic . flow chart showing patients progression through the study the study was registered with the umin clinical trials registry , study number umin000014 800 . patients were randomized on the day of oocyte retrieval into either the reduced needle ( rn ) group or the standard needle ( sn ) group , with a 1:1 allocation ratio . the needle used for the procedure was not known to the patient but for practical reasons it was known to the treating physician . all opu procedures were performed trans - vaginally , by a single physician , with ultra - sonography to guide the needle . patients were randomly assigned to one of the two groups on the day of oocyte aspiration . in each case , a single follicle with a mean diameter of > 17 mm was aspirated , using either a reduced needle with a 17 gauge body and a 20 gauge tip ( sense , vitrolife , sweden ; rn group ) or a standard 19 gauge needle ( uniever oocyte aspiration needle , japan ; sn group ) . all needles were connected to a syringe ( via a luer connection ) for manual suction , meaning that the aspiration pressure could be controlled as needed . if an oocyte could not be retrieved following puncture , follicle flushing was performed up to 5 times . pain was rated by each patient immediately after the opu by means of a visual analogue scale ( vas ) , comprising a vertical line from 0 cm ( no pain ) to 10 cm ( unbearable pain ) ( 5 ) . all patients were asked by the same medical doctor to mark the vas with their rating of the overall pain for the whole procedure . the pain perceived by patients 30 min after the procedure , captured in the same way as overall pain , was a secondary endpoint . bleeding from the vaginal wall ( at the point of needle penetration ) was also evaluated after opu . bleeding  was defined as continuous signs of bleeding even after the application of direct compression for 30 s , and  no bleeding  defined as cessation of blood loss after direct compression . the in vitro fertilization ( ivf ) procedure used in the present study has been described previously ( 6 ) . the retrieved oocyte was fertilized by conventional insemination or intracytoplasmic sperm injection ( icsi ) three to five hours after follicle aspiration . the presence of two pronuclei with extrusion of the second polar body was taken as the evidence of successful fertilization . embryos were evaluated microscopically before transfer , and those with more than seven blastomeres and less than 10% fragmentation were classified as morphologically good - quality embryos ( mges ) ( 7 ) . embryos were placed transcervically into the uterus using a soft et catheter ( kitazato , japan ) ( 8) . pregnancy was recognized when the development of a gestational sac was evident from transvaginal ultrasound on the 21st day after embryo transfer . the level of pain associated with the procedures is higher when it is performed without anesthesia , and the average vas score for the primary endpoint  overall pain experience  was anticipated to be 5.0 cm with the standard needle . the reduced needle was expected to reduce the vas pain score by 1.2 cm ( 24% ) , a clinically significant difference . forty - four patients would be needed in each study group to demonstrate this difference with 80% power and a significance level of 0.05 ( two - tailed tests ) . fisher s exact test was used to analyze dichotomous data and student s t - test and wilcoxon test when normality distribution was rejected were used for continuous variables pain was rated by each patient immediately after the opu by means of a visual analogue scale ( vas ) , comprising a vertical line from 0 cm ( no pain ) to 10 cm ( unbearable pain ) ( 5 ) . all patients were asked by the same medical doctor to mark the vas with their rating of the overall pain for the whole procedure . the pain perceived by patients 30 min after the procedure , captured in the same way as overall pain , was a secondary endpoint . bleeding from the vaginal wall ( at the point of needle penetration ) was also evaluated after opu . bleeding  was defined as continuous signs of bleeding even after the application of direct compression for 30 s , and  no bleeding  defined as cessation of blood loss after direct compression . the in vitro fertilization ( ivf ) procedure used in the present study has been described previously ( 6 ) . the retrieved oocyte was fertilized by conventional insemination or intracytoplasmic sperm injection ( icsi ) three to five hours after follicle aspiration . the presence of two pronuclei with extrusion of the second polar body was taken as the evidence of successful fertilization . embryos were evaluated microscopically before transfer , and those with more than seven blastomeres and less than 10% fragmentation were classified as morphologically good - quality embryos ( mges ) ( 7 ) . embryos were placed transcervically into the uterus using a soft et catheter ( kitazato , japan ) ( 8) . pregnancy was recognized when the development of a gestational sac was evident from transvaginal ultrasound on the 21st day after embryo transfer . the level of pain associated with the procedures is higher when it is performed without anesthesia , and the average vas score for the primary endpoint  overall pain experience  was anticipated to be 5.0 cm with the standard needle . the reduced needle was expected to reduce the vas pain score by 1.2 cm ( 24% ) , a clinically significant difference . forty - four patients would be needed in each study group to demonstrate this difference with 80% power and a significance level of 0.05 ( two - tailed tests ) . fisher s exact test was used to analyze dichotomous data and student s t - test and wilcoxon test when normality distribution was rejected were used for continuous variables next , they underwent opu with either the rn or the sn ( figure 1 ) . the number of patients enrolled was lower than estimated in the sample size calculation . patients baseline characteristics and data relating to the opu procedure are summarized in table 1 . the average age in the rn group was 40.83.9 years , compared with 40.33.6 years in the sn group . the mean number of retrieved oocytes was 0.970.16 in the rn group and 0.970.16 in the sn group , while the percentages of mature oocytes were 86.5% and 91.8% in the two groups , respectively . overall , there were no significant differences between the two groups in baseline characteristics or opu procedure data . baseline characteristics and oocyte pick - up ( opu ) procedure data in the reduced needle ( rn ) and standard needle ( sn ) groups meansd ; ns : not significant there was no significant difference in fertilization rate between the rn group and sn group ( table 2 ) . the pregnancy rate was numerically higher in the rn group ( 33.3% vs. 16.6% ) , although this difference did not reach statistical significance ( table 2 ) . assisted reproductive technology ( art ) outcomes in the reduced needle ( rn ) and standard needle ( sn ) groups mges : morphologically good - quality embryos ; ns : not significant the results relating to pain are shown in table 3 . the mean vas score for overall pain experienced during opu was 3.22.0 cm in the rn group , significantly lower than the one in the sn group 4.92.2 ( p<0.01 ) . there was no significant difference in pain experienced 30 min after opu between - groups . the frequency of bleeding from the vaginal wall after opu was reduced by 26.3% in the rn group compared with the sn group ( p<0.05 ; table 3 ) . pain and frequency of bleeding in the reduced needle ( rn ) and standard needle ( sn ) groups meansd ; ns : not significant another strategy is to use a thinner needle for the procedure . in the present study , overall pain during oocyte aspiration was significantly lower with a newly designed  reduced  needle with a thinner tip , in comparison with the standard needle . no negative effects were found with the rn group in relation to oocyte fertilization , embryo morphology , or pregnancy rate . it is difficult to evaluate pain experience during opu , because many factors can have an influence such as fear of pain , anxiety , physician s skill , operation time and technical factors including needle diameter and sharpness during opu . analgesia is another potential factor , although that was not the case here because analgesic was not used . in a previous study of the rn , all patients received local anesthetic using paracetamol and had paracercivcal block with lidocaine ( 4 ) . moreover , all patients received ovarian stimulation , meaning that several follicles were aspirated and the total opu time was longer than the time in the present study . under such conditions , it may be difficult to evaluate accurately the effect of the rn on pain . in the present study , it was hoped that the effect of the rn might be more obvious because patients had only one follicle aspirated and no analgesic drugs were used . in addition , to avoid possible bias related to multiple operators , all opus were performed by a single physician in the present study . consequently , a reduction in vas pain score was observed from 4.9 to 3.2 cm , a much larger relative reduction than in the previous study where corresponding vas scores were 2.6 and 2.1 cm . too much reduction of the needle diameter risks damaging the oocytes , although few studies have investigated this issue . with bovine oocytes , it has been demonstrated that both the diameter and bevel of the needle , as well as the aspiration vacuum , affect the recovery rate and developmental competence of oocyte ( 10 , 11 ) . previous investigation of the rn used in the present study concluded that human oocytes are unaffected by using a needle with a 20 gauge tip and an aspiration pressure of 90120 mmhg ( 4 ) . previously , a prototype needle similar to the rn investigated in this study was created . like the needle reported here , the prototype had a luer connection , a 17 gauge body and a 20 gauge tip . the oocyte recovery rate with this needle was comparable with that of a conventional needle . however , approximately 50% of retrieved oocytes had degenerated and it was concluded that the prototype needle was harmful for oocytes ( data not shown ) . this may be attributable to the way in which the prototype was made : where the two constituent needles was joined , the inner wall would not have been smooth and the flow of fluid passed this point would potentially have been turbulent . since the study was performed by a single physician on single - follicle oocyte retrievals with no analgesic treatment , patient perceived pain and physician perceived bleeding could be appropriately studied . still , one potential limitation of the current study was the relatively small number of the patients recruited . however , statistical analysis demonstrated that the study had sufficient power to detect an important difference . it was also thought that the patients would like to change our clinical practice quickly , taking into account the outcome of the study . another potential limitation of the study was that the type of needle was not blind to the physician . however , since the technique and average opu time were the same in both groups , it is unlikely that this would impact the results . this needle is not only useful for single - follicle oocyte retrievals in patients with no analgesic treatment but also more broadly in patients with fear of aspiration and those with a low threshold for pain . it will also be useful for patients with a history of bleeding and patients undergoing fertility preservation in conjunction with blood cancer treatment such as leukemia or lymphoma . patient perceived pain during single - follicle oocyte retrieval without analgesic was significantly lower with the newly designed aspiration needle with a thin tip than with a standard needle . in addition , post - procedural bleeding from the vaginal wall was lower with the rn .