To my best knowledge (of course it can also be outdated easily), progesterone does not have direct effect on embryo as suggested in the source text (it is very confusing by saying “使胚胎变得紧密”--embryo does not really grow tight/compact or increase in density I believe), embryo simply grows/expands once it finalised the implantation/nidation process.

I can only think of one implication of “紧密” is that progestrone does play a great role to prepare endometrium for implantation (a very short sensitive window opportunity) which means it thickens the endometrium, increases the blood supply via enlarge the vessels and it slows down the spontaneous contraction of uterus...etc, all these are to help the fertilized ovum to successfully implant and to enhance the early development of the embyro. A sufficient amount of progesterone/or a good response from endometrium to progesterone should provide a good lining fo uterus when nidation take place and the further growth of embryo--hence to have the embryo stick to the uterus wall, probably tightly. In other words, ‘紧密’here probably refer to the closeness of embryo and endometrium (literally, they grow together).

P.S: I do hope this is not a 'back-translation' to value the quality of the original Eng>Chi translation, otherwise, a correct back-translation would not reflect how wrong the original one actually was.