In certain species of sea life, male pregnancy is common. The male incubates and births live young instead of the female. Human males are not able to do so, and medical risks of a male pregnancy are extreme. Transgender individuals can become pregnant if reproductive organs are left intact. A common phenomenon is a sympathetic pregnancy, where a man exhibits all the symptoms of pregnancy along with his expectant partner.
Seahorses and pipefish are famous for male pregnancy. After a courtship with their monogamous mates, the female deposits her eggs into a pouch on the male's abdomen. The eggs are fertilized and remain in the pouch until the babies are fully developed. Infant seahorses look like miniature adults and are completely independent at birth.
Men could theoretically sustain a pregnancy intra-abdominally, but the medical risk is high. The placenta would attach to organs and can't be removed or separated, causing infection, hemorrhage, or death. Hormones used to support the pregnancy would probably cause serious side effects in both the father and baby.
While male pregnancy is unlikely, men may be able to feed the babies. Male lactation is observed in individuals with medical disorders involving prolactin, the hormone responsible for milk production. Certain heart medications have resulted in lactation, and pituitary tumors can also produce this side effect.
Female-to-male transgender individuals have become pregnant and given birth while living legally as men. This is possible if the female reproductive organs are intact and hormone treatment is interrupted. US resident Thomas Beatie, whose wife Nancy is infertile, is perhaps the most well-known. The Beaties have three children conceived with donated sperm that Thomas gestated. Researchers are working on developing a male egg from stem cells that would allow gay male couples to produce a child that is genetically theirs, using a pregnancy surrogate.
Fetus in fetu is not strictly a male pregnancy, but instead a parasitic twin that has become encapsulated within the body before birth. When the separation of an egg into identical twins goes awry, either conjoined twins will result or one twin will be absorbed by the other. The non-viable mass can appear in any part of the body, causing pain and serious problems if it grows. Most cases of fetus in fetu are found while the baby is still very young.
While many men say they would carry a baby for their partner if they could, men with Couvade's syndrome, known as sympathetic pregnancy, feel they actually are. They gain weight, feel irritable, and suffer from nausea and insomnia along with their pregnant partners. Pain can be experienced when the actual labor begins. A psychological explanation most researchers favor is that men feel marginalized during their partners' pregnancies or wish to spare them some of the discomfort.