Ms Pender was last seen at her flat on Church Street in town around 6 a.m. on Friday, August 23, 1996.
Her disappearance has been the subject of an ongoing investigation by garda stations in Tullamore and Portlaoise, but no one has ever been brought to justice.
Ms Pender was 25 years old and seven months pregnant when she vanished.
She was dressed in bright colours and white leggings and stood 1.65m (5’5″) tall with long blonde hair.
Gardai have asked anyone who met or saw Ms Pender or has information about her disappearance but has not spoken to officers to come forward.
They also urged anyone who had previously come forward but felt unable to provide garda with all of the information they had on the case to contact the investigation team again.
Anyone with information, no matter how minor or insignificant, can contact Portlaoise Garda Station on 057 867 4100, the Garda Confidential Line on 1800 666 111, or any garda station.
