ja, da kann man schon unsicher sein; denn man kann ja erwarten, dass im Normalfall Eheleute ein gemeinsames Kind haben.

the biological child of both partners in the marriage

"man kann ja erwarten, dass im Normalfall Eheleute ein gemeinsames Kind haben" - schon mal was von der "Patchwork-Familie" gehört? Der "Normalfall" ist doch heut schon fast die Ausnahme ;-)

anbieten würde sich "joint", aber ist wahrscheinlich unpassend, ich würde auch das "gemeinsame" eher weglassen. their child.

In a legal sense you can also say the natural child (of both partners). See definition:
http://www.yourdictionary.com/natural-child

As you can see here, the law in various countries used to assume what Hausfreund does.. but they cottoned on some time ago...
http://encyclopedia.jrank.org/LAP_LEO/LEGITIM...
http://www.jstor.org/pss/1327172

Or as the priest said in his sermon, it illustrates the difference between faith and knowledge: Mrs Murphy knows that all seven children are her children, but Mr Murphy believes that all seven are his own. ;-)

Just to leave it fluid: the couple's child. :-))

A married couple's child in common.

Here is an example from a legal document using "biological child of both husband and wife" and one from a legal advice site using "natural child of both spouses."

It may sound awkward having to use so many words in English, but unfortunately "joint" doesn't work in this meaning. This is used in expressions such as "joint child custody".

unless there's reason to be very specific, I'd go with Helmi's "the couple's child." It's a natural. :-)

http://www.google.at/search?hl=de&as_qdr=all...

Child-in-common certainly gets more google hits from relevant sites, but a closer analysis shows that these are almost all from the US. (.uk 16, .nz 4, .au 7.)

I believe Alexalex needs it for a legal translation, so it needs to be unambiguous.

eheliches Kind = legitimate child

Ich wuerde "biological child" waehlen, da es eindeutig ist.

"legitimate child" ist ein eheliches Kind, also eines, das in einer Ehe geboren wird. Es ist aber durchaus moeglich, dass der Vater des Kindes nicht der Ehemann ist!