Hi and welcome to HCM!
Thanks for posting your query here.
Endometrium is the inner lining of uterus
. Under the hormonal
influence of oestrogen and progesterone
the thickness of the endometrial lining changes. In a menstrual cycle the endometrial thickness is thin-1-4mm in the first four days where there is a period, followed by 4-14mm thickness which is called proliferative phase, followed by 15-28mm thick in the secretory phase. Now she is in the proliferative phase. After this there will be changes in the layers and will cause favorable environment for the embryo to get implanted. Gestational sac
is the implanted embryo with a wall around it.
If the gestational sac is not there that means she is not pregnant.
Hope this is helpful to you.