A transgender pregnancy is a pregnancy by a transgender person. Transgender subjects can be either trans men or trans women.
A trans woman is a male-to-female transgender person who was assigned to be a male at birth and has female gender identity. Trans women usually have XY-chromosomes and no uterus.
A trans woman usually does not have the necessary anatomy to allow fetal development. Thus, a trans women cannot become pregnant unless she receives a uterine transplant which has not yet been successfully performed in XY-subjects. Theoretically at least and with much medical intervention a pregnancy in a trans woman is possible by having an embryo implanted in the abdomen and being given the right hormones to support the pregnancy. This is called an abdominal pregnancy. In addition, theoretically a trans women could receive an ovarian transplant.
An abdominal pregnancy is a very dangerous and life-threatening medical condition. An abdominal pregnancy in a trans woman has never been attempted because it can potentially kill the person. Intentionally creating an abdominal pregnancy whether in a woman or a man is therefore unlikely to be studied in humans. In a transabdominal pregnancy the placenta usually attaches to internal organs such as the bowel, bladder, or liver, a condition generally referred to as "abdominal pregnancy".
A trans man is a person assigned to be a female at birth and who has a male gender identity. Trans men usually have XX-chromosomes and a uterus and ovaries, unless these were surgically removed. Thus, a trans man with a uterus and/or ovaries can become pregnant, and carry a baby to term, and there are multiple reports of transgender pregnancies in trans men.