Only way that I know DI will pay on maternity is Dr's order of bedrest. Yet with the common 90 day elimination period that would make it unlikely for it to ever pay. You can buy the 30 day elimination period but, the price jumps significantly.
You are thinking of LTD. Group STD usually has an elimination period of 1-2 weeks and will pay for maternity. 6 weeks is pretty standard payment for regular maternity, but I think there are a few states where the law says 8. Of course, for a bed-rest patient, the short-term DI lasts until needed or until the usual 180 day max.
For these group plans, most companies have a minimum number needed, and very high participation requirements.