. Admittedly, I only get to ride a couple of times per week but my instructor rides my horse 3 times per week (well, that usually includes one lesson for me but if I can't then he rides all three times) so she is kept exercised. You just have to be a bit more organised than before having a child lol.. I have a 17mth old, work 70%, and have a horse on DIY, and I manage. But you do have to make compromises. My horse doesn't get pampered or have as much time spent with her anymore (with regard faffing and generally being around her for hours on end), and it's a case of getting what is needed done, and leaving. It's lovely though in the sense that I am mucking out, enjoying what I always have with my horse, and have my gorgeous daughter there with me who loves my mare to bits already. 
It was very cute!
I basically then left my baby in the car (parked outside the stable) whilst I sorted my horse each day. Obviously that didn't include riding and any time I wanted to ride it would be by arrangement that my mum would have my baby (or OH obviously when he was off at weekend). She just used to sleep in the car when I mucked out. Just timed it around her feeds and changes etc. If she needed changing or feeding whilst I was there though I just did it there in my car (bottle fed). Now she's older, she just comes with me in the pushchair and sits at the door of the stable watching Peppa Pig on my iPhone.
I also work part time and irregular hours. It's a hell of a lot easier then I was expecting tbh. But then my YO is fantastic with kids and my mum is retired and does grandma duties if I ever want horsey tlc.Exactly this! I've got it down to an art! I pick my timings and then my 11month old will sleep in the car usually giving me enough time to muck out in the morning and then school or lunge. Admittedly I only tend to hack at weekends when OH can have her, but I don't mind, we school in the week and take it easy with lovely long hacks at the weekends
To be fair, I have had a very understanding OH and a fantastic instructor who has made many things possible which otherwise would have been a major stress.
Obviously depends on how you keep your horse, (livery/at home etc), whether you plan to return to work after mat leave/what sort of job you have and what your support network is like. 
I used to school or lunge while the baby was asleep, the monitor worked from his bedroom to the paddock.
It is easy while they are really small because they sleep so much and can't get anywhere (ie into trouble) without help, so as they get more active and time consuming you gradually adapt.
. Some of this is simply getting out of the habit; but also lack of facilities and the fact that he had so much time off. I chose not to ride when pregnant. Mind you, winter didn't encourage me to get on board, and now I'm itching. The main issue is prioritisation. For instance if my DH has already looked after him before work so I can feed them and poo pick, or even have a lie in, I can't expect him to also let me ride. I breastfeed so it's only recently that I've felt like there would be a solid window to go for a hack but it's not possible unless DH is at home. Many people have mentioned their mum and honestly without a non-working helper, it's going to be very very hard. I thought I had it sussed as my mum was made redundant, but looks like she has another job lined up. Boo My DS is 7 months, and I've been on my horse twice since he was born
I should say although horses are at home, I have no arena so really can't ride when baby sleeps. I don't think the monitor would stretch to a hack!