If you have submitted your OT request, pregnancy can certainly affect it -- in a temporary manner.
While on TCat, your medical limitations can affect your ability to complete trades training in the new occupation -- thus rendering you incapable of becoming qualified in that new trade until after your return to full duties.
A review of your medical file will occur during the OT process, and if you do not meet the minimum med cat required to undergo training in the new trade it will be rejected. In your case, as the MELs are only temporary ... you'd experience a temporary delay (IF they accept your OT in the first place ... nothing says that even were you not pregnant that it would go through).
As OT messages can be cut with very short notification of roll-over date, I'd be quite surprised if an OT application were accepted for a member who was not capable of fully performing all requirements of the occupations basic level trades trg. As an OT --- you'd be undergoing QL3, and most QL3s entail PT requirements, some QL3s entail heavy lifting (such as my trade) etc (you'd be limited to 10 lbs) and ergo would not be able to fully complete the course to the minimum standard.
Careers are aware of TCats/MELs, I'd think that rather than you having to turn down an OT offer --- you'd see it not even being offered until after you were returned to full duties which would enable successful completion of the new trades trg.