Table 1 Summary of the articles that fulfilled the inclusion criteria

Junqing et al., 2002 [5]

Young men from seven geographical areas

China

Not mentioned

Geometric mean 55.45 million per mL

93.8% for semen density

Mean 59.89%

81.3% for sperm with forward progression

Mean 2.61 mL

81.9% normal semen volume

89.1% for total sperm count

98.8% for normal sperm morphology

Median 70.0%

65.3% for sperm viability

Participants whose sperm met all WHO standard parameters accounted for 42.3%

Li et al., 2003 [6]

College students in Chengdu area

China

Not mentioned

50.90 ± 27.31 million per mL

Grade A and B sperm was 42.21 ± 15.38%

Grade A sperm was 29.48 ± 13.71%

2.61

Sperm viability rate was 56.40 ± 14.77%

The volunteers with normal sperm accounted for 62.84%

Volunteers with abnormal sperms 37.16% (90.67% with abnormal motility, 19.21% with abnormal concentration, 10.78% with both abnormal concentration and abnormal motility)

Sperm concentration and sperm viability rate have a tendency to decrease

Toft et al., 2004 [7]

Recently proven fertile men from four regions

Greenland

Not mentioned

Median 53 million per mL

Median 60%

Median 3.2 mL

Median 185.6 million

Sperm cell concentration and total sperm count in Greenland seems to be in the lower range compared to studies from Europe, the USA, and Japan

Ugwuja et al., 2008 [8]

Male partners of the infertile couples

Nigeria

Not mentioned

70% of the study population had low sperm count

high defective parameters (64%), mainly asthenozoospermia and teratozoospermia

Li et al., 2009 [9]

General healthy population

China

2007

Median 77.8 million per mL

Median 70.9%

Median 33% for sperm rapid progressive motility

Median 52.6% for sperm progressive motility

Median 2.3 mL

Median 167.7 million

61.1% of healthy males had at least one semen parameter below normal threshold values

A high proportion of healthy males in Chongqing area of southwest China had abnormal semen parameters values according to WHO criteria

Mukhopadhyay et al., 2010 [10]

Andrology laboratory

India

Two periods:

group 1: 1981–1985 group 2: 2000–2006

Did not change

Declined in the second group

Declined in the second group

Significant change in motility

Espinoza Navarro, et al., 2010 [11]

Healthy young males

Chile

Not mentioned

Sperm concentration was 62.8 million per ml, 76% had normal sperm concentration

Total motility was 42.2 ± 23.2%, 38% had normal overall motility

Grade A motility 19.2 ± 18.6%, 26% had normal grade A motility

Volume was 2.9 ± 1.6 ml, 72% had normal volume

82% normal total sperm count

Normal morphology 15.0 ± 7.9%, 57% had normal morphology

64% had normal vitality

Ugboaja et al., 2010 [12]

Male partners of the infertile couples

Nigeria

January– December 2006

68.0% had semen fluid abnormalities

30.0% had single factor abnormalities

38.0% had combined factor anomalies

16.7% asthenozoospermic

14.7% asthenooligozoospermic

13.2% asthenooligoteratozoospermic

1.4% azospermic

High rate of semen fluid abnormalities among the male partners

Xu et al., 2011 [13]

Male infertility patients

China

Not mentioned

42.39% abnormal

27.35% were found normal in all seminal indexes

The incidences of asthenospermia 37.31%

The incidences of oligospermia 8.94%

Mainly, decreased motility

Zou et al., 2011 [14]

Military personnel from different geographical areas

China

2007–2009

39.4 million per mL

0.439

15.8% for sperm rapid progressive motility

30.1% for sperm progressive motility

3.0 mL

120.1 million

62.5% according to WHO recommendations (2010)

88.3% had at least one semen parameter below normal values

Men had markedly lower mean sperm concentrations, sperm counts, and sperm motility

Owolabi et al., 2013 [15]

Male partner of infertile couples

Nigeria

2004–2008

25.6% oligozoospermic

18.5% teratozoospermic

11.5% asthenozoospermic

6.2% azoospermic

3.2% oligoteratozoospermic

2.3% oligoasthenozoospermic

2.1% oligoasthenoteratozoospermic

0.9% asthenoteratozoospermic

Mendiola et al., 2013 [16]

University students

Spain

Current study period 2010–2011 and the results were compared to a previous group 2001–2002

Declined

Median declined from 51.0 million per mL to 44.0 million/mL

Declined

median 149 million to 121 million

Total sperm count and sperm concentration may have declined in young Spanish men over the last decade

Jiang et al., 2014 [17]

Close to general population

China

2007–2012

Median was 62.0 million per mL

Sperm concentration decreased from 66.0 million per mL to 49.0 million per mL

Median 39% progressive motility

Median 2.4 ml

Median 10.5% for normal morphology

percentage of sperm normal morphology decreased from 13.5% to 4.5%

Incidence of azoospermia was increasing

There is a decline in semen quality of adult males in Sichuan, China

Mendiola et al., 2014 [18]

Unselected young men

New York, USA

Not mentioned

Median total sperm count was 158 million

23.1% sperm concentration below 20 million per mL

15.8% sperm concentration below 15 million per mL

Median total sperm count was 158 million

Li et al., 2014 [19]

From a hospital

a control group from a sperm bank and another infertility group

China

Not mentioned

Asthenospermia

Azoospermia Oligoasthenospermia

Rao et al., 2015 [20]

University students

China

2010–2013

50.2 million per mL

decrease in sperm concentration during the 4 years observation

0.586

3.0 ml

148.1 million

0.85

Birdsall et al., 2015 [21]

Sperm donors

New Zealand

From 1987 to 2007 continued in the period 2008–2014

The declination did not continue

Declined in period 2008–2014

Semen quality has not changed in New Zealand men over the last decade

Ugwa et al., 2015 [22]

Male partner of couples presenting with inability to conceive

Nigeria

Not mentioned

52.38% normospermic

26.98% azoospermic

20.64% oligospermic

60.3% asthenospermic

kim et al., 2015 [23]

Men presenting for reproductive issues

Korea

January 2002–December 2003, January 2007–December 2008, and January 2012–December 2013

40.1 ± 52.3 million/ml

28.2 ± 27.7 million/ml

2.8 ± 1.8 ml

46.1 ± 35.6 normal morphology

No significant changes in the semen parameters of Korean men from 2002 to 2013

Borges Jr. et al., 2015 [24]

Males undergoing conventional seminal analysis

Brazil

Group1: years 2000 to 2002

Group2: 2010 to 2012

Decreased from 61.7 million to 26.7 million

Decreased significantly from 183.0 million to 82.8 million

Decreased significantly from 4.6% to 2.7%

The incidence of severe oligozoospermia significantly increased from 15.7% to 30.3%

The incidence of azoospermia increased from 4.9% to 8.5%

A significant time-related decline in semen quality of infertile patients

Huang et al., 2016 [25]

Sperm donors

China

2001–2015

Decreased from 68 to 47 million/mL

Decreased from 34 to 21 million

Decreased from 31.8% to 10.8%

The semen quality among young Chinese men has declined over a period of 15 years

Centola et al., 2016 [26]

Sperm bank

USA

2003–2013

Decline

Decline

No change

Decline

Decline in semen quality

Erenpreiss et al., 2017 [27]

General population

Baltic countries (Estonia and Latvia) and Lithuania)

2003–2004

63

11–15% of men had low semen quality, 37–50% intermediate and 38–52% high semen quality

Ajayi et al., 2017 [28]

Fertility Centre

Nigeria

Group 1: 2003

Group 2: 2013

Decreased

group 1: 34.6

group 2: 21.8

Decreased

group 1: 47.9%

group 2: 45.3%

Decreased

group 1: 50% good, 44% fair, 2% poor, 4% no motility)

group 2: (15% good, 81% fair, 4% poor)

Decreased

Group 1: 2.7

group 2: 2.6

Decreased

Increased

Mahmud et al., 2018 [29]

Infertility clinic

Bangladesh

2000–2016

Declined by 20%

Total rapid motility: declined by 20%

Azoospermia increased by 18% between some ranges

Karabulut et al., 2018 [30]

3 infertility clinics

Turkey

2011–2016

Azoospermia cases (5.85%)

Cryptozoospermia (8.73%)

Elbardisi et al., 2018 [1]

48 different nationality from a hospital

Qatar

2012–2015

32.3

45.4

25.1

79.9%

azoospermia (6.05%),

oligospermia (23.3%),

asthenospermia (30.5%),

teratospermia (48.7%)

Rahban et al., 2019 [31]

General population

Switzerland

1980–2014

48

17% of men had sperm concentration below 15 million/mL

25% had less than 40% motile spermatozoa,

43% had less than 4% normal forms

deterioration

Li et al., 2019 [32]

Hospital

China

2011–2017

increased

40.1–52.1

Progressive motility:

increased

33.4–38.1

Increased

117.8–153.1

Vahidi et al., 2020 [2]

Clinical center for infertility, comparison between two groups

Iran

Group 1: 1990–1992 Group 2: 2010–2012

Increase

Grade A: decreased

Grade B: increased

Grade C: constant

Grade D: constant

Decrease

47.88% of infertile men showed normal semen parameters

No deterioration

Morey-León et al., 2020 [33]

An assisted reproduction center

Ecuador

2017–2018

A high percentage of patients presented sperm morphology and quality values below the reference limits

27.4% normospermic

27.9% teratozoospermia

8.8% oligoteratozoospermic

Al-Kandari et al., 2020 [34]

One center study

Kuwait

Not mentioned

8.2% cases had normal semen tests

30.2% oligoasthenospermia

Siqueira et al., 2020 [35]

Infertility clinic

Brazil

1995– 2018

Reduction of 0.24 million/mL each year

Reduction of 2.84 million/year

Reduction of 0.52% each year

Barrera et al., 2020 [36]

Sperm bank: same subjects

Uruguay

1989– 2017

Declined in the 28 years

Increased in the 28 years