The short version is that a Changeling can shapeshift to a form that can produce sperm to impregnate someone, and can shapeshift to a form with a uterus and eggs to get pregnant, but a pregnant Changeling cannot shapechange to a form without a uterus without ending the pregnancy.
In the Worlds of Star Trek: Deep Space Nine novella Olympus Descending, Odo learns from an elder Changeling that there are no "infant" Changelings, but just Changelings that are kept separate from the rest of the Great Link, as Changelings cannot reproduce since the departure of the ancient Progenitor centuries ago; ...
Having said that: while changelings CAN mate with other humanoids, I'd say that it is RARE for them to impregnate creatures of other species. It can happen, but the fertility rate isn't that high. It's quite possible that humans and other human-compatible species are the most viable. With that said…
A changeling can alter his or her gender (and reproductive abilities) as part of using the race's minor shapechange ability. Two changeling parents breed a changeling child. A changeling and a humanoid can mate to produce an offspring with the child having a 50% chance to be of either race.
Are they biologically asexual and just choose their current sex with shapeshifting? Yes. What's been stated in the past is that changelings set their sex with shapeshifting.
"A human child might be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often it was thought that fairies exchanged the children.
Changelings have the unique ability to transform their body to look however they want, provided they keep the same arrangement of limbs they already possess. It is very rare to actually see a changeling in it's 'true' appearance. Even when disguised, changelings can instinctively recognize each other.
Changelings born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
Obviously, a changeling's appearance can change at any moment, but they do have a true form. Their true skin is usually grey or pale white, and their facial features are blank slates to an unnerving degree. They're typically thin and gaunt, and their hair is usually silvery with an occasional green or pink tint.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
Lifespan. Changelings reach maturity slightly faster than humans and live as long as humans do, typically about 100 years.
Within this description there are variants of changelings that show subtle differences. Some have opalescence in their skin (as if it contains various colors instead of just being white) and their eyes can be completely black.
A changeling can't make the wings at all. If they had wings, like from a class feature or something, then they could look like an Aaracokra with wings because they will have those limbs at that point.
The Changelings were sent to retrieve Picard's body so that they could specifically remove parts of his parietal lobe—the part that was supposedly defective from Irumodic Syndrome.
According to legend, the abducted human children are given to the devil or used to strengthen fairy stock. The return of the original child may be effected by making the changeling laugh or by torturing it; this latter belief was responsible for numerous cases of actual child abuse.
As crew members beamed on and off their ships, the computers recognized this new genetic code as "common biology" and integrated it into the DNA of every officer in the fleet. The Changelings wanted Jack because of they needed his Borg-altered DNA, though they later ended up acquiring this from Picard's body.
Changelings are typically born with female anatomy, but some prefer to maintain male primary and secondary sex characteristics once in control of this ability.
Sometimes changelings are old, nasty fairies who revel in the sorrow they cause; or fairies with prodigious appetites for human food or mortal breast milk. Sometimes the changelings are fairies so old and worn out that their kinfolk have left them behind, happy to be rid of them in exchange for a plump human child.
The true form of a changeling is a bit paler than a typical human with slightly elongated limbs. Their eyes are colorless and ringed by black as pictured in the artwork above by Julia Zhuravleva. They have no body hair, but the hair on their head can be platinum, white, or silver.
Changelings have red blood like a majority of the other races. Bleeding around others may alert many that the Changeling is not of the same race (should the blood color vary). For this reason, a Changeling will usually choose to take the form of a race with red blood.
Two changeling parents breed a changeling child. A changeling and a humanoid can mate to produce an offspring with the child having a 50% chance to be of either race. Your question noted that there's a 50/50 chance of getting a changeling if they mate with a human...but it's not just human.
If you're Irish, you know your child is a changeling if it's sickly and doesn't grow in size like a normal child. Another fairly obvious sign is if it grows a beard or has long teeth. Finally, your child might be far more intelligent than other kids that age, and possess unusual insight.
In some cases, the changeling grows up often being very loyal and caring to friends and family despite what they are. They often care more for their friends and family than they do themselves. Most changelings enjoy helping as they get older, but they often feel different, out of place.
Shapeshifting – Changelings can appear human, and are capable of shifting between forms in order to feed. Sharp teeth – Changelings have a lamprey-like mouth covered in sharp teeth that allow them to feed on humans. Super strength – Changelings have superior strength compared to humans.
Age. Changelings mature slightly faster than humans but share a similar lifespan—typically a century or less. While a changeling can transform to conceal their age, the effects of aging affect them similarly to humans.