Background In spite of the last decade increase in availability of contraception, around half of the annual 21 million pregnancies notified in low- and middle-income countries in individuals aged 15-19 years are unintended. We sought to explore the contribution of the underuse of modern methods of contraception (MMC) to the annual incidence of unintended pregnancies among adolescent women. Methods We used Demographic and Health Survey (DHS) data from 12 low- and middle-income countries. The pooled analysis exploring the risk of unintended pregnancy included 7268 adolescent women with a current unintended pregnancy and 121894 currently not pregnant 15- to 19-year-old sexually active women who did not desire pregnancy. For each country and the pooled analysis, the odds ratio of unintended pregnancy was calculated in relation to the type of contraception (MMC, Traditional Methods, and No Contraception). Expected unintended pregnancies and population attributable fraction (PAF) of unintended pregnancies attributable to not using MMC were calculated for each country. Results The use of traditional methods was associated with a 3.4 (95% confidence interval (CI)=2.1-4.7) time increased odds of having an undesired pregnancy compared with the use of MMC of contraception while not using any method of contraception was associated with a 4.6 (95% CI=2.6-6.6) times increased odds. The population attributable fraction (PAF) of not using MMC accounted for 86.8% of the estimated unintended pregnancies (9464654 in total in the 12 countries) in the pooled analysis. PAF ranged from 65.8% (1022154) for Bangladesh to 95.1% (540176) for Niger and the estimated number of unintended pregnancies because of the use of traditional methods or non-use of contraception ranged from 18638 in Namibia to 4303872 in India. Conclusions Eight million out of 9.5 million unintended pregnancies occurring annually in twelve countries could have been prevented with the optimal use of MMC of contraception. MMC need to be further supported in order to further prevent unintended pregnancies globally.