Here’s some information I’ve found on this. From Original Plumbing:
If a trans guy were to become pregnant and continue on testosterone, the use of T may cause problems with maintaining the pregnancy due to competition with estrogen and progesterone; thus if one wanted to keep the pregnancy, he would definitely have to stop his testosterone, and instead, may have to take replacement female hormones. This would definitely need to be done under the care of a obstetrician.
And from South Wind Women’s Center (PDF):
If you suspect you may be pregnant, have a pregnancy test as soon as possible, so that your doctor can stop your testosterone treatment, which may be dangerous to the fetus. If you want to become pregnant, you must first stop your testosterone treatment and wait until your doctor tells you that it is okay to begin trying to conceive. Your doctor may check your testosterone levers before clearing you to begin efforts of conception.
Overall, it seems that taking testosterone during pregnancy is inadvisable, but as always, consult a medical professional about this.