[SammoJ]

So I had always heard this is the case, and now believe this must be a common misconception. See Table 6 in this file from the Office of National Statistics:

http://www.ons.gov.uk/ons/publications/re-reference-tables.h...

While from 2004 to 2012 total UK fertility increased from 1.80 to 1.98, the fertility rate for non-UK born women actually /decreased/ from 2.50 to 2.29 (albeit in a slightly messy non-monotonic fashion) while the fertility rate for UK born mothers /increased/ from 1.69 to 1.90 quite monotonically.

It would be interesting to see the stats including 2nd and 3rd generation immigrant mothers (i.e. born in UK both with parents born elsewhere).

[eliben]

I wasn't aware of UK-born mothers' increased fertility rate.

However, I'll argue that even if the fertitily rate of immigrant mothers decreased, this can explain the data because if more immigrants came in, the average fertility rate goes up (given that immigrants' decreased rate is still quite a bit higher than UK-born).