Ah, Teacher Bai's mini-classroom is in session:
When seeds get startled, due to the characteristic of cell differentiation, some seeds will split early on, becoming identical twins/triplets/quadruplets/quintuplets, and so on.
The resulting fetuses from this kind of situation are mostly of the same gender, with very, very few being of different genders.
There's also a type called fraternal twins, which are fetuses formed from different seeds, with the likelihood of them being the same gender or different genders each at fifty percent; dragon-phoenix twins usually come from fraternal seeds.
Generally, women determine if they are pregnant based on their menstrual cycle.
But for giants like Xiaozhai, their periods cease during the Innate phase, and they also come equipped with a whole range of internal diagnostic technologies like ultrasound, color ultrasound, X-ray, MRI, and more.