I do a LOT of pregnancy massage. I usually have at least one new pregnant client a week. Some of those are week 38 some of those are week 12. The Wk 38 ones I may only see once or twice, the early ones much more often.
Every now and then I consider doula training, but being a doula is so disruptive to your schedule - you need to drop everything for someone's labour, so you need to reschedule all those people you have booked in. It's very hard to balance, it seems to be almost impossible to fit in a normal business schedule with the births. Doulas often don't have to rely on their doula money to cover the basics (partners income etc cover that). All similar things to being a midwife.
I have found with pregnancy massage that it is cyclic, some months I will only get the early pregnancy ones, some months I will have all of the early pregnacy ones that have been coming for 6 months wanting an appointment every week for the last month and there will be 10 other late pregnancy inquiries. 9 months after Christmas/New Year silly season is one of the peaks. So I have some quiet months and I have some ridiculously busy months, it balances out. I wouldn't want to put those busy months on hold due to doula services (which would be around the same peak demand times).
Some clients do continue after they have the baby (I often offer a free or half price post natal massage (depends on how much I have seen them during the pregnancy) that helps get them back) but time and money become more of an issue. I usually do the post-natal as a home visit, home visit pregnancy massage clients are more likely to continue after they have had the baby. Post natal visits often take longer due to interruptions for feeding/distressed partners not coping/mother not coping with hearing the feeble attempts at settling the baby. Often the first post-natal treatment is the first time they relinquish care of the baby to someone else for an hour - it can be very stressful for everyone involved.
I couldn't rely on only pregnancy/post natal work for income, it varies too much (and I am in a baby boom area, if you're in an area of mainly retirees it would be impossible) you still need some regular non-mother clients. Also it means you don't have regular clients all year, they come during the pregnancy and you don't see them again, until the next pregnancy. I've got a few clients on their 3 & 4 pregnancy coming to me who I don't see between pregnancies at all, but they've come to me every pregnancy.
I like clients dropping off and getting new ones all the time, but you can get the same high turnover with chronic pain clients. Some people like having the same clients year in year out without many new clients coming in, you can't do that with pregnant clients, it's a continual turnover of clients, which can be a bit unpredictable and hence a bit stressful during quiet times.