The childbirth was kept an absolute secret.
Mikoto was accompanied by an infant (Sasuke), and in front of Kushina were two friends.
Kushina, like with Mikoto, the childbirth will be painful, according to one of the women.
As the child came forth, the Kyuubi probably came out (the seal weakened) that's why Mikoto also witnessed it.
Kushina was in pain.
Several Anbu members were on standby in front of the room.
Date of birth: October 10.
The seal weakens. Minato [tries] to strengthen it.
A masked ANBU takes the infant (Naruto).
The masked man says, "Let's get away from the human sacrifices, child, otherwise you also will be one"
AND
Why did Kushina become a human sacrifice?
It seems that Whirlpool country (Hidden Village of the Whirling tides) clanspeople were masters of the sealing technique.
The First Hokage's wife MIto was also a human sacrifice.
Mito was the first human sacrifice.
By dying, Kushina became one.
Kushina died, with Naruto being the 3rd.
For the human sacrifice to happen, Konoha and the Whirling Tides had to be friendly countries and distant relatives
Even now, it's Konoha's best back mark (design on the back of the suit)
Naruto's Jersey's back design is the swirl of the Whirlpool country (Whirling Tides' Mark).
Now, that sealing jutsu, the special chakra is aimed at being destroyed.