My fiance's ultrasound indicate she's 5 weeks pregnant & with yolk sac but no corpus luteum. can this be treated? can baby survive without it?

Progesterone: The corpus luteum is a hormone producing part of the ovary that is pumping out Progesterone to allow implantation and prevent menses. It functions maximally during the first four weeks and then tapers off by 10 weeks as the placenta takes over hormone production. The fact that she is at 5 weeks means the corpus luteum did it's job! your doctor may measure Progesterone and supplement if needed.

Yes: You can't always see a corpus luteum by ultrasound and does not matter if you do not. Have a repeat ultrasound in 1-2 weeks to confirm that the baby is present ( called a fetal pole) and that there is a heart beat.

Yes: The corpus luteum is a cyst that forms on the ovary on the side where the egg that became fertilized came from. It typically provides Progesterone to support the baby early on before the placenta forms, but fairly often they are not seen and pregnancy progresses normally. The yolk sac has a similar job. At 5wks, a follow-up scan is needed to document normal growth and a heartbeat (7-8 wks).