Alice Foss-Myers

 is the unborn daughter of Henry Foss and Erika Myers. She is also the great-niece of Erika's paternal aunt Dr. Lillian Myers.

Birthday
In, it's assumed Erika had just found out she was pregnant, rushing over to the Old City Sanctuary from England to tell Henry in person that he's going to be a father based on her having some "urgent news" to tell him that "couldn't wait".

Later in the episode, Magnus uses the specific terms "embryo" and "fetal" to describe the state of the developing offspring. An "embryo" marks 6 weeks gestation. A "fetus" marks 8 weeks gestation until birth.

Near the end of the episode, Magnus says that Erika's gestation rate will change from being 10.5 months long (presumably the duration of pregnancy for a HAP, being 1.5 months longer than a normal human's 9 months) to now be up to possibly 22-months-long (more than twice the normal length for a HAP).

Considering that Erika was at least roughly 8 weeks pregnant = ~2 months along already and considering the max estimated changed gestational rate for a new total overall pregnancy length of 22-months-long, it means that Alice will presumably be born 20 months after "". This episode is known to take place 3 weeks after the Hollow Earth Invasion which occurred in early 2011 in January / February.

If everything went well and Erika had a healthy full-term pregnancy just as Magnus had expected it to be, then Alice would have likely been born in September / October of 2012.