***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Purpose The aim of this study was to determine if the size of zona pellucida thinning area by laser assisted hatching could affect the rates of pregnancy and implantation for vitrified-warmed embryo transfers at the cleavage-stage . Methods A total of 120 vitrified-warmed cleavage-stage embryo transfers were r and omly assigned to either quarter or half of zona pellucida thinning group . Results The rates of clinical pregnancy ( 46.7 versus 25.0 % ) and implantation ( 32.0 versus 16.2 % ) were significantly greater in the half thinning group than in the quarter thinning group ( P = 0.0218 and P = 0.0090 , respectively ) . Conclusions The results of this investigation show that , in vitrified-warmed embryo transfers at the cleavage-stage , the size of zona pellucida thinning area by laser assisted hatching impacts the rate of clinical pregnancy and implantation and that half of zona pellucida thinning significantly increases both of these results compared with quarter of zona pellucida thinning From 1993 to 1994 , in our centre , laser-assisted hatching was performed on 2- to 4-cell stage embryos obtained from in-vitro fertilization ( IVF ) patients . We treated 376 embryos from 96 patients with repeated IVF failures ( two to four attempts ) ( group A ) and 397 embryos from 111 patients undergoing IVF for the first time ( group B ) . Embryos were transferred immediately after the laser treatment . Both groups were compared to control groups ( A ' and B ' ) whose embryos were transferred with intact zona pellucida ( ZP ) . The result ing clinical pregnancies were 41 in A and 44 in B versus 24 in A ' and 23 in B ' respectively . The pregnancy rates per patient were 42.7 and 39.6 % versus 23.1 and 19 % in the control groups ( P < or = 0.05 ) , while the implantation rates per embryo were 12.2 % in A and 11.8 % in B versus 7.3 % and 7.1 % . These results show that laser zona thinning of human embryos at 48 h after egg retrieval significantly increases the implantation and pregnancy rate ( P < or = 0.05 ) Assisted hatching by zona drilling using acidic Tyrode 's solution was performed during three r and omized trials in 330 in-vitro fertilization patients . The trials were design ed in order to study the overall effect of the procedure and whether characteristic patient [ i.e. maternal age and basal levels of follicle stimulating hormone ( FSH ) ] and embryonic features ( i.e. zona pellucida thickness ) are important for the decision to perform assisted hatching routinely . Couples ( n = 137 ; Trial 1 ) in whom the female partner had normal basal FSH levels were r and omized in a  control group ( without micromanipulation ) and a zona drilling group ( all embryos micromanipulated ) . The incidence of implantation ( 67/239 ; 28 % ) of zona-drilled embryos compared favourably with that of control embryos ( 49/229 ; 21 % ) , but the difference was not significant . Retrospective analysis revealed that those embryos whose zonae were thicker than 15 microns were rescued . In order to test the validity of this finding , selective assisted hatching was performed on embryos with a poor prognosis in 163 other patients ( Trial 2 ) . The couples were r and omized into a control group and a group in which embryos were selectively zona-drilled , based on zona thickness and other embryonic features . The rate of embryonic implantation in the selectively zona-drilled group was 25 % ( 70/278 ) , significantly ( P less than 0.05 ) higher than that of control embryos ( 51/285 ; 18 % ) . Although it was demonstrated retrospectively and prospect ively that assisted hatching by zona drilling is effective in embryos with thick zonae ( greater than or equal to 15 microns ) , patients whose embryos have thin ( less than 13 microns ) zonae may be jeopardized by the procedure . ( ABSTRACT TRUNCATED AT 250 WORDS Two hundred eighteen consenting patients entered a r and omized study of the application of chemical zona pellucida thinning on their day 3 embryos , prior to uterine transfer . Of those control patients ( n = 108 ) , whose embryos remained unmanipulated , 40 ( 37.0 % ) have ongoing/delivered pregnancies , while in the experimental group ( n = 110 ) , whose embryos had their zonae pellucidae chemically thinned , there are 49 patients ( 44.6 % ) who have ongoing/delivered pregnancies . Although this difference is not significant , clearly the application of this micromanipulative intervention has not been detrimental , and this bodes well for routine application of embryonic micromanipulation procedures in general . Certain patient subgroups were studied including older women , those with elevated basal follicle stimulating hormone levels , patients with embryos of differing zona thickness , and patients with embryos of differing uniformity of zona thickness . No significant influence of chemical removal of the outside of the zona on the implantation rate of embryos in any of these subgroups was observed other than a marginally significant ( P = 0.095 ) improvement of implantation of embryos with less than 4.0 µm variation in zona thickness when chemical zona thinning was applied . Failure of chemical zona thinning to enhance human embryo implantation significantly , compared to assisted hatching by complete zona drilling , strongly suggests that the bilayered human zona pellucida needs to be fully breached , unlike that of the mouse OBJECTIVE To examine whether opening of the zona pellucida ( i.e. , assisted hatching ) accelerates implantation . DESIGN In a controlled , r and omized trial , patients were assigned to  control  and  assisted hatching  groups . SETTING All patients studied were of the Center for Reproductive Medicine at Cornell University Medical College . INTERVENTION All patients underwent stimulation with gonadotropins after luteal phase GnRH down regulation  . Assisted hatching with zona drilling using acidic Tyrode 's solution was performed on the assigned embryos . MAIN OUTCOME MEASURES Luteal E2 , P , and hCG on days + 5 , + 6 , + 7 , + 8 , + 9 , + 11 , + 13 , and + 15 were measured . The implantation time , peak midluteal E2 and intervals between these two values were studied . RESULTS Implantation occurred significantly earlier in the assisted hatching group . The interval between implantation and peak midluteal E2 was also significantly shorter in the assisted hatching group than in the controls . However , there was no significant difference in the day of the peak midluteal E2 between the assisted hatching and control groups . CONCLUSION Assisted hatching may enhance embryo implantation not only by mechanically facilitating the hatching process but also by allowing earlier embryo-endometrium contact . Such early contact may enhance embryonic development potential and may optimize synchronization between embryo and endometrium , result ing in improved implantation efficiency OBJECTIVE To assess the effect of laser hatching on human embryo damage and subsequent development using the Zona Infrared Laser Optical System ( ZILOS ) . DESIGN R and omized controlled study . SETTING Tertiary care fertility clinic . PATIENT(S ) One hundred fourteen donated and discarded frozen human embryos . INTERVENTION(S ) Embryos were thawed , cultured with cleavage and morphology evaluated periodically , and r and omized into control , partial hatching , or complete hatching groups . The laser hatching procedure was performed by ZILOS .  Zona thickness and embryo diameter were recorded . Complete hatching involved the production of a full-thickness defect in the zona and partial hatching , a defect in the outer half of the zona . No laser treatment was administered to the control group . MAIN OUTCOME MEASURE(S ) Blastocyst development and completion of hatching process . RESULT ( S ) No significant difference was noted between the three study groups for their baseline characteristics . There was no significant difference in blastocyst development among the three groups . However , the complete hatching group showed a significant increase in hatching compared to the control group . CONCLUSION ( S ) Complete laser hatching of human embryos using the ZILOS does not have an adverse effect on subsequent development and increases the rate of completion of hatching Preliminary data from a prospect i ve r and omized study of assisted hatching ( AH ) of thawed human blastocysts demonstrated that the rate of complete hatching was significantly higher for AH performed near the inner cell mass ( ICM ) than for AH at the side opposite to the ICM . This suggests the existence of polarity in the hatching process in human blastocysts , which requires further evaluation Laser-assisted hatching is little documented in the literature regarding its efficacy in cryopreserved-thawed ( CT ) embryo transfer cycles . The aim of the present study was to evaluate in a r and omized manner the efficacy of thinning one quarter of the zona pellucida of CT embryos to a depth of 50 - 80 % of the original thickness , via laser treatment ( the qLZT-AH procedure ) , in improving implantation and pregnancy rates .  Two population s were studied : population I , patients who had all their supernumerary embryos cryopreserved , regardless of their morphology , and population II , patients at risk of ovarian hyperstimulation syndrome who had all their embryos cryopreserved . Artificial and natural protocol s were used for the embryo transfers . A total of 350 laser-thinned CT embryos were compared with 352 intact zona embryos . No difference in implantation or pregnancy rate was found after using qLZT-AH in either population . These findings suggest that qLZT-AH should not be routinely performed in cryopreserved embryo programmes 
    Output:
    
SUMMARY ANSWER No , unlike laser zona opening , zona thinning does not facilitate embryo hatching .
We postulate that zona thinning is not beneficial in human embryos .
WIDER IMPLICATION S OF THE FINDINGS The experiments demonstrate that zona thinning is not equivalent to zona opening for assisted hatching .