Nutrition, lifestyle factors and non-infectious diseases like diabetes, cancers or cardiovascular problems, each accounted for about 10 percent of stillbirths.
About 7,200 babies are stillborn every day -- some 2.6 million per year -- and half of these deaths occur during delivery, according to a quintet of studies published by The Lancet on Tuesday.
The figures for 2015 represented a meagre drop from around 24.7 to 18.4 deaths for every 1,000 total births from 2000 to last year, the medical journal reported.
The overwhelming majority of stillbirths, about 98 percent, occur in low- and medium-income countries.
"But the truly horrific figure is 1.3 million" stillbirths that occur during delivery, The Lancet editors Richard Horton and Udani Samarasekera wrote in a comment.
"The idea of a child being alive at the beginning of labour and dying for entirely preventable reasons during the next few hours should be a health scandal of international proportions. Yet it is not."
For the purposes of the study, stillbirths were counted as foetuses lost during the final three-month trimester, or after 28 weeks of pregnancy.
Deaths before this cutoff are termed miscarriages.
The series found that prolonged pregnancy -- delivery several days beyond the estimated birth date -- was the main cause of stillbirths, contributing 14 percent.
Next in line were maternal health problems.
Nutrition, lifestyle factors such as obesity or smoking, and non-infectious diseases like diabetes, cancers or cardiovascular problems, each accounted for about 10 percent of stillbirths.
Malaria infection accounted for about eight percent of stillbirths and syphilis 7.7 percent, the analysis showed.
An estimated 6.7 percent of stillbirths was attributed to the expectant mother being older than 35, and 4.7 percent to eclampsia -- a serious condition of pregnancy that can cause seizure-inducing high blood pressure.
Rich, poor gap
Sub-Saharan Africa had more stillbirths than any other region.
Given the slow rate of improvement, "over 160 years will pass before the average pregnant woman in sub-Saharan Africa has the same chance of her baby being born alive as does a woman nowadays in a high-income country," the study said.
But the series also highlighted wide gaps between rich and poor people even in high-income countries.
A poor woman in a wealthy country has about double the risk of stillbirth than a rich one.
"Stillbirth rates for women of south Asian and African origin giving birth in Europe or Australia are two-to-three times higher than white women," said a statement.
The country with the lowest rate, with 1.3 stillbirths per 1,000 total births, was Iceland, and Denmark was next at 1.7 per 1,000.
They were followed by Finland, the Netherlands, Croatia, Japan, South Korea, Norway, Portugal and New Zealand.
The worst performer, out of 186 countries measured, was Pakistan with 43.1 stillbirths per 1,000 total births.
The rest of the bottom 10 were Nigeria, Chad, Niger, Guinea-Bissau, Somalia, Djibouti, Central African Republic, Togo and Mali.
In 2014, the World Health Assembly -- the world's highest health policy body -- endorsed a target of 10 or fewer stillbirths per 1,000 total births by 2035.
But the Lancet series found the average annual rate of reduction, at two percent, was far slower than for maternal deaths (three percent) or deaths of children under five (4.5 percent).
The series was comprised of five research papers compiled by more than 200 authors, investigators and advisers from 43 countries.