Over the past few weeks it has got considerably worse.

Danke Azzurra!

@ #1 Müsste es nicht "it has gotten worse" sein?

@ 3
Soweit ich weiß, ist "gotten" AE. Ich muss BE schreiben, dort ist "got" sicherlich korrekt.

Why not go with the direct translation? No obnoxious helping-verb participles!

But 'rather' isn't really right here, sorry.

'Deteriorated somewhat' is possible, but I understood 'ziemlich' to be stronger in this context than just 'etwas.'

'Pretty much' is grammatically possible, but also pretty casual/colloquial.

More suggestions:

In the last/past few weeks ...
Over the last/past few weeks ...

... his/her condition has ...
... he/she has ...
... things have ...

(if it's a serious or terminal illness -- I hope that's not the case)

... taken a turn for the worse. (useful idiom)
... deteriorated considerably.
... gone pretty sharply downhill.

(for either a serious or not-so-serious condition, like a chronic or minor ailment)

... got(ten)/become/grown considerably/a good deal worse.