The study also found Nigerian girls had become better educated
Unplanned pregnancies among Nigerian teenagers and young women have risen despite improvements in educational levels, a report found.
In 2003, 16% of pregnancies among girls and women aged 15-24 had been unintended, compared with 10% in 1990.
The study from the New York-based Guttmacher Institute said low use of contraceptives was partially to blame.
The institute said Nigerian authorities had failed to promote sexual health information for young Nigerians.
It found the proportion of adolescent women with some secondary education had increased 16 percentage points (from 34% to 50%) between 1990 and 2003.
But over the same period the use of modern contraceptive methods among sexually active adolescent women in Nigeria had changed very little, from 4% to 8%.
Also in that time, the proportion of sexually active young women who knew where to access family planning services nearly halved, from 32% to 18%, found the report - which analysed health data from Nigerian authorities and non-governmental-organisations.
Nearly one-third of sexually active women aged 15-24 had had an unmet need for modern contraception in 2003, found the study.
"We are failing Nigerian adolescents when it comes to providing them with the information and services. They need to delay marriage and avoid unintended pregnancies," said the report's co-author, Professor Friday Okonofua.
She said this was leaving young women at risk from unwanted pregnancies and sexually-transmitted diseases, such as HIV.
The study also noted the population of Nigeria - Africa's most-populous nation, with 150 million people - was growing by 2.2% annually, and at that rate would double every 32 years.