In vitro fertilization and Mahatanhasankhaya Sutta

[onmitt]

How is In vitro fertilization possible if according to Mahatanhasankhaya Sutta "...the descent of the embryo occurs with the union of three things..."?
http://www.dhammatalks.org/suttas/MN/MN38.html

[santa100]

From MN 38:

Bhikkhus, the descent of the embryo takes place through the union of three things. Here, there is the union of the mother and father, but the mother is not in season, and the gandhabba is not present—in this case no descent of an embryo takes place. Here, there is the union of the mother and father, and the mother is in season, but the gandhabba is not present—in this case too no descent of the embryo takes place. But when there is the union of the mother and father, and the mother is in season, and the gandhabba is present, through the union of these three things the descent of the embryo takes place.

Notice the highlighted parts, those indicate that the key to the descent of the embryo is not in the physical presence of the mother, but in whether her eggs are available at the moment or not. Similar deduction can be made for the father part. His presence isn't the key part but his sperm is. He can show up but if he's not "engaging" with the mother then obviously nothing's gonna happen. In vitro pretty much does the same thing for there's the "in season" element (ie. availability of the eggs), the "union" element (ie. the lab guy merges the eggs and sperms on the petri dish), and all that's remained is the presence of the gandhabba, which should be no problem for they should be everywhere, not just inside the bedrooms where couples conduct their private business.

[onmitt]

... all that's remained is the presence of the gandhabba, which should be no problem for they should be everywhere, not just inside the bedrooms where couples conduct their private business.

This is the part that I don't understand. How "the being, driven by kamma, who will take birth on that occasion" gets drawn to a lab without future mother and father present.

[santa100]

This is the part that I don't understand. How "the being, driven by kamma, who will take birth on that occasion" gets drawn to a lab without future mother and father present.

Notice the being can't just pick his future destination as s/he wishes. His/her kammic stream does. Otherwise everyone can just pick their most beautiful, most healthy, most intelligent, and most wealthy parents for their next birth. Therefore, the being, regardless of its own preferences, will be placed in whatever environment that is most suitable for the manifestation of its specific kammic stream. One thing a person can do is to conduct himself in accordance with the Dhamma and kamma will naturally steer him toward a suitable direction accordingly.