Q&A: My doctor said the membrane hasn't attached to the sak properly. What does this mean?

It's always hard to know exactly what is meant when a doctor translates a medical problem for a patient, who then goes on to translate it again for another doctor. But I'll try...

The "membranes" are made up of an amnion and a chorion, two layers. These fuse after 12 weeks, making up the gestational sac. This sac continues to expand as the baby grows, touching the inner wall of the uterus by 16 weeks, filling the uterine cavity. If you are truly 9 weeks, then the only thing I can figure is that either your doctor is seeing the separation between the amnion and chorion, with some fluid between, or a combined chorioamnion which hasn't slammed up against the uterine wall yet. It would be normal for both of these situations before 12 weeks.
I would think that your doctor has an excellent ultrasound machine that can pick up such detail. The reason for the repeat ultrasound in three weeks is that by that time, the amnion membrane and chorion membrane should have merged into the "chorioamnion". Unless there are other troubling signs (lack of a fetus or fetal heart rate), it sounds like there's no problem here. You should ask your doctor the significance of his or her remark in your individual situation.