About 170 people are set to climb the Brecon Beacons peak on Saturday.
The washing line is visiting well-known sites across the UK throughout June.
The chief executive of Sands, the stillbirth and neonatal death charity, said she wanted people to talk about the "taboo subject" of baby death.
Dr Clea Harmer said the campaign was also aimed at "provoking debate and discussion as to why do so many babies die and what can be done to reduce this number".
The charity said the UK was three times slower than some European countries at lowering the baby death rate and families were often sent home with poor explanations of why their baby died.
A babies remembrance service is to be held for bereaved parents at the Wenallt Chapel at Thornhill Crematorium in Cardiff on Sunday.
A washing line pegged with 15 babygrows is to be carried up Pen Y Fan to raise awareness of the number of babies who die shortly before, during or after birth in the UK each day.