Pregnant women are having C-sections without anaesthetic while others are forced to use scraps of tents in place of period products, aid workers say.
Babies are being "delivered into hell", without their mothers having any anaesthetic, according to UNICEF communications specialist Tess Ingram.
With more than 1.9 million Gazans displaced since 7 October, a tent "city" of almost two square miles has been set up in Rafah, south of the strip.
No food or medical supplies are allowed to enter the territory, including sanitary towels, tampons and contraceptive pills that could stop or delay periods.
Riham Jafari, an ActionAid worker based in Bethlehem, said: "Imagine having to manage your period with no period products, toilet paper or soap, and no chance of being able to wash yourself.
"This is the reality for hundreds of thousands of women and girls in Gaza right now.
"Some women cut part of towels to use on their periods - this is not healthy.
"They are using parts of the tents or fibres. They cut off part of it to use as pads and some of them are using their extra clothes as pads."
Using unclean materials as sanitary products can cause a risk of infection and potentially deadly toxic shock syndrome.
Ms Ingram added: "Seeing newborn babies suffer while some mothers bleed to death should keep us all awake at night."
Women and girls have made up roughly 70% of the 25,000 people killed in Gaza since 7 October, according to the Hamas-run health ministry.
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One ActionAid Palestine worker, who asked to remain anonymous, added: "There was no water available for me to get clean during my period. I had no sanitary pads for my own needs."
A woman called Adara, who was forced to flee her home with her four children, said they "suffer a lot whenever we want to go to the bathroom" and have to "stand in line for a long time".
With clean running water incredibly sparse in Gaza, people there are falling well short of the 15 litres needed per person per day.
According to estimates from the UN's agency for Palestine, only one of the three water pipelines between Israel and Gaza is functional, and there is only one toilet per 486 people.
ActionAid is calling for a ceasefire to allow more aid into the territory.