Becky, nice belly pic!
Thanks for sharing!
Tkh: the reason why so many clinics prescribe progesterone, injections or vaginal inserts (pessaries) is because in a natural pregnancy, there is an ovulation. The egg is released in your tube and the follicle stays behind in your ovary. If the egg is fertilized, the embie starts producing HCG and the follicle, now called the corpus luteum, does not die. That's how your body gets the message that you're pregnant. The corpus luteum will send messages to your brain so it keeps producing progesterone (instead of switching to estrogen and make AF start). Given that the placenta won't be fully mature until week 10-12, the corpus luteum has to stay alive that long to make sure that progesterone is still produced in sufficient amounts.
During IVF, both the follicle and egg are retrived. So, given that there are no follicle left behind in the ovary, it can't turn into a corpus luteum and do its natural job. That's one of the main reasons why doctors will prescribe progesterone right after egg retrieval, because your body hasn't ovulated and the placenta isn't mature enough. Taking progesterone during the first trimester also reduces the risks of miscarriage. Some docs feel comfortable stopping the progesterone at 10 weeks, while others play it safe and wait until week 12. It depends on many factors. Be prepared, though... it's very common to have brownish discharge a couple of days after you stop taking progesterone. It's absolutely normal and it's not the baby bleeding. It happened to me, lasted a couple of days, and then stopped. No big deal. Had I not been warned, I wouldn't have appreciated the surprise...
I guess the logic of Becky's doctors is that, given that many follicles are artificially stimulated and not retrived during the procedure, some will hatch and there will be at least one corpus luteum in her ovaries, which will be kept alive by the HCG her baby(ies) is/are producing. So, even if her placenta(s) is/are not mature yet, her body produces enough progesterone. But maybe Becky was told another explanation?