My error. Not a big one; but seeking clarification is always good. I meant to say "Their menstrual cycle will cease, but the ovaries still function with the gamete (egg) still released once a month." By the way, 'menstruation' and 'ovulation' are different.
Either way, I'm saying the initial process still functions despite a major component of the plumbing no longer being present after an hysterectomy and the egg is still released into the system once a month. This is true for women; and a parallel may be true for Queen Aliens, too.
With this in mind, it's not outside the realm of credibility for the Queen to still be capable of reproduction without the egg sack as the egg itself may form further inside its body and then travel to the eggsack where it then develops further inside this organ to a fully formed egg.
Speculating here, but in order to completely sterilise the Queen, removing the egg sack may not be enough. You may need to remove whatever passes as gonads it has inside its body. And because it's Alien, who knows what it's actually capable of doing?
But if you genuinely want an explanation of how that egg possibly got aboard the Sulaco, then thee presence of the Queen and further speculation about its biology and reproduction is as good a place to start as any.
After all, who really does know everything there is to know about the Queen's reproductive system?
Especially when there's such a abundance of weird sexuality on our own planet. Male pigs have a corkscrew penis. Tapirs have a penis that is half their body length*. Male cats have a barbed penis. Female kangaroos have 3 vaginas. Just try and imagine the orgy if these 4 animals get together on a Saturday night after too many beers. After all, they're not all that far removed from us. They're all mammals.
-Windebieste.
*I mean seriously huge. So big in fact, I figure female tapirs must be hollow all the way up to the shoulders. ...and no. I'm not going to post images. You can google that one for yourself. I'm sure tapir penis was the inspirational design for 'Prometheus's hammerpede. lol.