World and Olympic 800m champion Caster Semenya's battle to stop a rule change affecting female athletes with high testosterone levels could see her miss "the bulk" of the 2019 outdoor season.
Governing body the IAAF intended to bring in the new rules on 1 November but has put that back to 26 March 2019.
It wants to wait for the outcome of a legal challenge from Semenya first.
The new rules aim to restrict the levels of testosterone in female runners.
They will apply to women in track events from 400m up to the mile and require that athletes have to keep their testosterone levels below a prescribed amount "for at least six months prior to competing".
So the delay means female athletes with high testosterone - of whom South African Semenya is the most notable - will not be allowed to run for six months from the date the rule changes come in.
That would take them to 26 September - just one day before the 2019 World Athletics Championships begin in Doha.
Semenya, and Athletics South Africa (ASA), are challenging the rules at the Court of Arbitration for Sport (Cas).
IAAF Health and Science department director Stephane Bermon said the governing body recognised the five-month shift "could result in affected athletes having to sit out the bulk of the outdoor season leading up to the World Championships, including international competitions such as the Diamond League, which begin in May 2019".
The IAAF's proposed rule would mean some female runners with naturally high testosterone levels would have to race against men or change events, unless they took medication to control their levels.
IAAF president Sebastian Coe said Semenya and ASA had agreed on the delay as part of a deal to get the legal case settled as quickly as possible.
"We have agreed not to enforce the regulations against any athlete until the contested regulations are upheld," he added.
"In exchange, they have agreed not to prolong the process. All athletes need this situation resolved as soon as possible."
In a statement, the IAAF added it "remains very confident of the legal, scientific and ethical bases for the regulations, and therefore fully expects the Court of Arbitration for Sport to reject these challenges".
ASA said it was "very pleased" with a decision that meant the changes were "effectively suspended pending the outcome of the appeal".
It says the rules will have a "discriminatory effect on female athletes like Semenya" and believes "the medical data relied upon by the IAAF is flawed".
Semenya, a two-time Olympic champion and three-time world champion, has previously been asked to undertake gender testing by athletics chiefs, but no results have officially been made public.
"It is not fair. I just want to run naturally, the way I was born," said the 27-year-old South African recently.
Testosterone is a hormone that increases muscle mass, strength and haemoglobin, which affects endurance.