I Guess theres a relation between the reason for the high birth rate and the infant mortality rate. Since children dont survive, people in these countries tend to have more children

Countries with the Highest Birth Rates in the World
Rank Country Birth rate(births/1,000 population)
1 Niger 48.91
2 Uganda 47.45
3 Mali 47.29
4 Afghanistan 47.27
5 Chad 46.50
6 Somalia 46.04
7 Angola 45.14
8 Liberia 44.81
9 Congo, Democratic Republic of the 44.73
10 Burkina Faso 44.46
11 Malawi 44.35
12 Sierra Leone 43.34
13 Yemen 43.16
14 Benin 42.57
15 Guinea 42.26
16 Madagascar 41.91
17 Mauritania 41.79
18 Zambia 41.77
19 Sao Tome and Principe 41.36
20 Nigeria 40.87

Source: CIA World Factbook

Countries with the Highest Infant Mortality Rates in the World
Rank Country Infant mortality rate(deaths/1,000 live births)
1 Angola 192.50
2 Afghanistan 165.96
3 Sierra Leone 145.24
4 Mozambique 132.04
5 Liberia 130.51
6 Niger 122.66
7 Somalia 118.52
8 Mali 117.99
9 Tajikistan 112.10
10 Guinea-Bissau 108.72
11 Djibouti 105.54
12 Malawi 104.23
13 Bhutan 102.56
14 Tanzania 100.55
15 Nigeria 100.38
16 Burkina Faso 98.67
17 Ethiopia 96.93
18 Chad 94.78
19 Congo, Democratic Republic of the 94.69
20 Congo, Republic of the 93.86

Source: CIA World Factbook