Using the withdrawal method to prevent sperm reaching egg was one of the unreliable traditional contraception techniques, as was trying to avoid the most fertile times of the month.
The researchers found that 15 million of the unwanted pregnancies in the 35 low-income to middle-income countries surveyed might have been prevented if modern methods of contraception had been used.
The analysis, by epidemiologists working for the World Health Organization (WHO), is published in Human Reproduction, the journal of the European Society of Human Reproduction and Embryology.
The estimates were reached by extrapolating data from demographic and health surveys that were conducted between 2005 and 2012 across 35 countries, ranging from Azerbaijan in Asia to Zimbabwe in Africa.
The problems beneath the underuse of modern contraceptive methods have policy solutions, says one of the study's authors, Dr. Howard Sobel.
Speaking as the regional coordinator of the WHO's western Pacific reproductive, maternal, newborn, child and adolescent division, he says:
"National strategies need to be put in place to address unfounded health concerns, fear of side-effects, opposition, and underestimated risk of pregnancy.
These need to be coupled with good-quality contraception that is available and affordable."
Contraceptive methods were classified as modern, traditional or non-existent. Traditional methods included withdrawal of the penis before ejaculation, and calendar methods to avoid sex during the most fertile part of a woman's menstrual cycle.
Contraception classified as modern included the below (along with a technique that assumes infertility during the absence of periods experienced by women who exclusively breastfeed for the first few months of their baby's life - the "lactational amenorrhea" method):
- Combined oral contraceptives
- Progestogen-only pills
- Injectable contraceptives
- Intrauterine devices ("coils")
- Male and female condoms
The researchers found that, compared with the use of these modern methods, there was a 2.7 times greater likelihood of an unwanted pregnancy from traditional methods of contraception. Failing to use any method of contraception, whether traditional or otherwise, was associated with over 15 times the risk of undesired pregnancy.
Of an estimated 16.7 million undesired pregnancies every year in the 35 low- to medium-income countries, 15 million might have been avoidable with the correct use of modern contraception.
Instead, an estimated 13.5 million women did not use modern methods at all, and a further 1.5 million used them incorrectly.
The demographic and health surveys used for the data were globally standardized and involved trained interviewers in face-to-face meetings with women.
The 14,893 women in the surveys who did not want to get pregnant but failed to use modern contraception were asked to give one main reason for this.
Fear of side-effects or concern over health was cited by 37% - an answer that did not vary by category of wealth, although two thirds of these women had no high school education.
Opposition to contraception was the next main reason, cited by 22% of the women. This answer grouped opposition or religious prohibition from the woman herself, her partner or someone else.
Further reasons women in the surveys gave for not using modern methods of contraception included:
Dr. Sobel says the greatest concern, about health, was "not backed up by evidence."
The problem, he adds, is compounded by the way health workers take up an "important role to play in reassuring, educating, treating symptoms and finding the methods that best suit an individual." Dr. Sobel says:
"Frontline health workers need the skills to do this, and our experience has been that many have the same misconceptions.
We could prevent the overwhelming majority of pregnancies if we could debunk the myths and misperceptions about modern methods, and use long-term methods of contraception, such as implants and intrauterine devices."
Dr. Sobel looks to international efforts on women's healthcare. The aim of the UN was for universal access to reproductive health by 2015 when its "millennium development goals" were agreed 10 years ago, but: "Of all the health-related millennium development goals, universal access to reproductive health is the one that is most off-track," Dr. Sobel says.
Specific measures remain available to help turn this failure around "in the next era" of the UN goals, he says - and "strategic investment in methods like implants, though expensive upfront, are far less costly to families, governments and society than having larger families due to undesired pregnancies."