Table 3.

Results for telomere length and fertility.

Studies	Samples	Method of Telomere Measurement	Results—Main Findings	p
Clinical criteria of fertility
Thilagavathi et al., 2013 [22]	Sperm	qPCR	Infertile men: STL = 0.674 ± 0.028	<0.005 *
			Controls: STL = 0.699 ± 0.030
Turner et al., 2013 [23]	Sperm	Q-FISH	No association between male fertility and STL	>0.05
Yang et al., 2015 [25]	Selected sperm	qPCR	No significant association between STL and clinical pregnancy rates	0.90
Cariati et al., 2016 [27]	Sperm	qPCR	STL between 0.2–2.0: pregnancy rate = 35.7%	0.04 *
			STL < 0.2 or > 2.0: pregnancy rate = 0.0%
Mishra et al., 2016 [28]	Sperm	qPCR	Infertile men: STL = 0.609 ± 0.15Controls: STL = 0.789 ± 0.060	<0.0001 *
Biron-Shental et al., 2018 [30]	Sperm	FISH	Sub-fertile sperm: STL = 0.6 ± 1.2%Fertile sperm: STL = 3.3 ± 3.1%	<0.005 *
Lafuente et al., 2018 [32]	Selected sperm	Q-FISH	Previously achieved a natural clinical pregnancy STL = 26.17 ± 8.20 kb	0.024 *
			Couples who had never conceived STL = 19.50 ± 5.05 kb
Torra-Massana et al., 2018 [13]	Thawed and selected sperm	qPCR	No significant effect of STL on reproductive outcomes:
			Biochemical pregnancy rate	0.411
			Clinical pregnancy rate	0.986
			Ongoing pregnancy rate	0.769
			Live birth rate	0.595
Darmishonnejad et al., 2019 [34]	Sperm and leukocytes	qPCR	Infertile men: STL = 0.74 ± 0.15	<0.05 *
			Fertile men: STL = 1.24 ± 0.18
			Relative telomere length in leukocytes was no different between the two groups
Berneau et al., 2020 [35]	Selected sperm	qPCR	Clinical pregnancy: STL = 1.026 ± 0.013	0.188
			No clinical pregnancy: STL = 0.995 ± 0.016
Darmishonnejad et al., 2020 [36]	Sperm and leukocytes	qPCR	Fertile men: STL = 1.09 ± 0.13	0.002 *
			Infertile men: STL = 0.61 ± 0.07
			Fertile men: LTL = 1.1 ± 0.14	0.1
			Infertile men: LTL = 0.76 ± 0.07
Rocca et al., 2021 [40]	Sperm	qPCR	ART group: STL = 0.9 ± 0.3	0.02 *
			Control group: STL = 1.2 ± 0.6
Lopes et al., 2020 [37]	Selected sperm	qPCR	No relation between STL and time of infertility	0.556
			No influence of relative STL on implantation rate	0.508
ART embryo criteria
Yang et al., 2015 [25]	Selected sperm	qPCR	STL is positively associated with the good embryo quality rate	<0.001 *
			STL is positively associated with the transplantable embryo rate	<0.001 *
			No association between STL and fertilization rate	0.49
Torra-Massana et al., 2018 [13]	Thawed and selected sperm	qPCR	No significant correlation between STL and the average score of embryo morphology	0.08
			No significant correlation between STL and the fertilization rate	0.35
Darmishonneiad et al., 2019 [34]	Sperm	qPCR	Positive significant correlation between fertilization rate and STL	0.007 *
Berneau et al., 2020 [35]	Selected sperm	qPCR	STL is positively correlated with fertilization rate	0.004 *
			No significant association between STL and embryo cleavage rate	>0.05
Lopes et al., 2020 [37]	Selected sperm	qPCR	No influence of relative STL in fertilization rate	0.411
			No influence of relative STL on embryo cleavage rate	0.900
			No influence of relative STL on AB embryo grade rate	0.123
			No influence of relative STL on embryo fragmentation	0.136
			No influence of relative STL on blastocyst formation rate	0.836

Results are expressed as mean STL ± standard deviation of the mean. STL determined by a qPCR-based method could be expressed as absolute or relative mean telomere length or correlation coefficient. * p values < 0.05 are significant.