The poster, created in-house, read: "100,000 people are alive today because of our laws on abortion. Why change that?" It then featured a link to the group’s website, where an explanation of the number is available.
Fourteen people complained that the ad was misleading and questioned whether the 100,000 figure could be substantiated.
The number reflects the fact that the 1967 Abortion Act, which legalised abortion in Great Britain, was not extended to Northern Ireland. The laws in the country are still highly restrictive, meaning in most cases women are unable to get an abortion, though a number travel to Britain each year to do so.
Both Lives Matter submitted a lengthy explanation to the Advertising Standards Authority of how it had arrived at the number of 100,000, which it said was meant to be credible, but conservative.
To produce the figure, it had looked at data from both England and Wales, and Scotland. It examined two separate measures: the abortion rate (the number of abortions per 1,000 women) and the abortion ratio (the number of abortions per 1,000 live births).
Using various methodologies, the group produced estimates for the number of abortions that would have taken place in Northern Ireland since 1967, ranging from 247,331 to 139,379. It said that even if other cultural differences it had not taken into account lowered the figure by a further 25%, there would still have been more than 100,000 abortions.
In an equally long assessment, the ASA tore a number of holes in Both Lives Matter’s working. Using the group’s sums as a starting point, it made a number of revisions and came up with an estimated range of 91,488 to 106,144.
But the ASA ruled that while the figure of 100,000 may have inflated to some extent the number of people alive today whose mothers would have had an abortion had it been legal, the evidence showed a reasonable probability that the figure would be at least 100,000, which is what readers would expect.
As a result, the watchdog ruled the ad was not likely to materially mislead readers.