Ketones are caused by the body breaking down fat. This happens for either or both of 2 reasons. Firstly, insuffcient insulin; Insulin acts to control the breakdown of fats. Normally this breakdown goes on all the time but if there isn't enough insulin the process goes into 'overdrive' .
Secondly it the body doesn't have enough glucose for energy it will again make use of fat .This happens either in fasting (starvation) or for example during endurance exercise. In both cases the ketones production will stop if glucose is supplied
and sufficient insulin is present However any amount of ketones can cause some insulin resistance so more insulin than normal may be needed once ketones are present.
It is possible to get DKA pregnant or not with lower blood glucose levels. When not pregnant it is quite rare (n the west, less so in developing countries) and is usually asscociated with another illness (diarrrohea/vomitting). However,at one hospital they studied 20 subsequent cases of DKA presenting in pregnant women, of these 20, 2 of them were euglycemic (had blood glucose within 'normal' levels).
Montoro et al 1993
If you are pregnant then you should (according to the American Diabetes Association) check for ketones when you have high glucose levels and if you are ill. Though, that's no different to any other T1.
Managing Preexisting Diabetes for Pregnancy
Summary of evidence and consensus recommendations for care
http://care.diabetesjournals.org/content/31/5/1060.full
Obviously If present then you should seek advice from your doctor/midwife... or better still know what to to in advance...just in case.
Like Nigel's, my consultant doesn't think any level of ketones is a good idea, he instructs people on pumps (who are at risk of insufficient insulin in the case of non delivery) to test for ketones every morning, irrespective of glucose levels. I must admit I don't .
It did hit home how quickly they could appear when a cannula fell out during a run. Instead of my normal very low glucose level after runnning, I had relatively high levels and ++ on a urine strip.
I've also had a positve result (+) during a D&V bug, and that time glucose levels were low (hypo at one point). Throw insufficient insulin into the mix and those 'starvation' ketones could quickly develop into something less 'benign'. We had someone writing about that happening to them and the resultant DKA not so long ago.