Hmmm, I think a combination of magic and tech works best here. Perhaps a lead on an artifact that was uncovered by a corp-sponsored dig and leaked via a mole in the dig; allowing for the team to go after it... but they have stiff competition from not only the corp guards, but other teams looking for a payday, and even the local flora and fauna as an unexpected side effect of the artifact. The artifact in question is what would provide the heightened mana effect to allow the embryo's viability until it's implanted in the mother, and even then she'd probably have to visit it during the pregnancy at least once a week (or month depending on how strong I decide to make the artifact's effect) to keep the fetus healthy up until the final trimester.
Soo, perhaps the first trimester she has to bask in the artifact's influence on a weekly basis, second trimester is once a month, and by the time of the third trimester the fetus has grown strong enough that it no longer needs the aspected mana field to help sustain it. After that the team is left holding a hot potatoe that has an enhancing effect on animal and plantlife (combination of Beast, Plant and Fertility mana aspects is what I'm aiming at). Hmm, any suggestions for side effects that it could have on anything that's over-exposed to it? (mainly for the run to acquire it, I imagine that the team is going to make sure it's effects are as contained as possible once they have it in hand)
As possible side-effects I can suggest two:
-first is that after exposure to the artifact any sexual act which might result in a pregnancy, will result in pregnancy. Birth-control devices fail, infertility is temporarily cured, etc. This is also the power that would allow the mating between a human and a shapeshifter, but the downside is that everybody will get the next person they have sex with pregnant (or get pregnant themselves if they are female). You can build a time-limit in it (example: if you are away from the device for one month, it doesn't work anymore). As an FYI: I nicked this from Scion Roleplay, where you have a power which does exactly this.
-the second effect is that if you are too long in the device's vicinity, you might get SURGE'd. After a suitable time period in the device's vicinity (a day, a week, a couple of hours, whatever you feel like) they have to do a body+edge roll and on a glitch or crit glitch they will get the SURGE quality. As I don't know how that looks like in 5th edition, I can't give any advice on which powers you want to give (positive and negative, I would give some of both), but maybe go for a Satyr-like look for males and a nymph-like look for females.
I would suggest to have the second effect visible first on an NPC, so the players know something is up. They will then have to either accept the fact that this can happen, or find a way to block it (there should be a way, encasing it in lead is always a good way of doing this, or bringing it into a magic dead zone, although the second one might be a lot harder). The two who want to get pregnant will have to be in the effect and they will have to decide if they want to run the risk or not.