Aims: The first study investigated the agreement between embryologists when assessing cytoplasmic strings (CS) in human blastocysts. The second study investigated the idea that embryos originating from fresh ejaculate sperm is comparable to those originating from frozen-thawed ejaculate sperm. The third study investigated and expanded on the association between the presence of CS, blastocyst quality and live birth rate. The fourth study used timelapse technology to investigate whether the presence of CS in human blastocysts was associated with a clinical pregnancy. Methods: In the first study, CS activity in one hundred blastocysts was assessed to measure inter- & intra-observer agreement. In the second study, timelapse was used to observe embryo development in those originating from fresh ejaculate sperm compared to those originating from frozen-thawed ejaculate sperm. In the third study, timelapse was used to investigate the association between the presence of CS and their characteristics, with blastocyst quality, development and live birth in one thousand day 5 human blastocysts. In the fourth study, timelapse was used to assess CS presence in blastocysts to investigate the link between those blastocysts that formed a fetal heart after transfer. Results: From the first study, a moderate level of inter and intra observer agreement was observed when five embryologist assessed day 5/6 blastocysts for the presence of CS and their vesicles. From the second study, it was found that no differences were detected in key developmental events between embryos originating from fresh ejaculate sperm compared to their frozen-thawed counterparts. The third study confirmed that CS presence in human blastocysts is associated with blastocyst quality. The fourth study, the presence of CS in human blastocysts was found to be associated with a fetal heart. Conclusions: The first study showed a moderate level of inter- and intra-observer agreement when the presence of CS and their vesicles was assessed. The second study found that there are no differences in the morphokinetic parameters of early embryo development when either fresh or frozen ejaculate sperm was used for ICSI insemination. The third study confirmed that CS presence in human blastocysts is associated with blastocyst quality. The fourth study found that the presence of cytoplasmic strings in human blastocysts is associated with the probability of clinical pregnancy with fetal heart.