ruptur amnion.

amniorrhexis (Wikipedia)

Rupture of membranes (ROM) or amniorrhexis is a term used during pregnancy to describe a rupture of the amniotic sac. Normally, it occurs spontaneously at full term either during or at the beginning of labor. Rupture of the membranes is known colloquially as "breaking the water" or as one's "water breaking." A premature rupture of membranes (PROM) is a rupture of the amnion that occurs prior to the onset of labor.

Sometimes, a child is born with no rupture of the amniotic sac (no rupture of membranes).