Most insurance companies have a 'waiting period' for maternity cover so that you can't find out you're pregnant, sign up for cover and then charge all the costs to the company. DKV has the shortest waiting period of 8 months, most are around a year. That means that you sign up for them and you have to wait for 8 months to elapse before you can make any maternity related claims.
For example if you found yourself pregnant in the 5th month of the 8 month waiting period you would not be able to claim for your appointments, scans, drugs, tests etc in the first 3 months of your pregnancy but after the 8 months have elapsed you can claim for everything from that date forward.
That's how it works with DKV anyway, I asked lots of questions before I signed up with them.
Another good question to ask when you're researching insurance is to ask if they treat "successful" and "non-successful" (sorry, that's the term they seem to use) pregnancies differently, some have a much lower limit for the "non-successful" pregnancies but on occasion a miscarriage would require expensive medical intervention and horrible to find out you aren't fully covered if the worst happens. I think DKV treats both the same (which is how it should be).
Also, ask what coverage the baby gets immediately after birth and how soon they need to be insured under their own policy - important if they are born with a condition requiring treatment. What their policy is for twins and multiple births? Is their limit per pregnancy or per year (important if you have two very close together ;-), and if they have a separate limit for complications e.g. emergency c-section.
Good luck with your research!