Thanks you lot, this is my first post and I already feel like I'm talking with a bunch of friends.
I cannot be sure but I think my MIL knows I like to go out for a couple of hours every weekend and calls DH to find out when. Its good you asked because now I am thinking if I don't tell DH in advance about my plans, MiL will not know lol
Now that you are asking, I also remember that when I first met her years back, I was struck by how negative she was towards women in general, and DH's elder brother's wife (my sister in law SIL?) in particular. She has no sisters /daughters etc and would often say she is glad she has no female in her life etc. She had already fallen out with my SIL and they were not on speaking terms. All my poor SIL had done to irritate her was to sit on the sofa between MIL and DH's brother when they were talking! My MIL (and my DH sided with my MIL back then) immediately saw that as jealous behavior, called her rude and since that day made it a point to criticise SIL on everything. She is according to MIL a bad wife and mum. I found SIL very reasonable and so when we got married I asked DH to visit my MIL for a few hours at least once a week 'without me'. I thought that will reassure her that I'm not taking her son away. Whenever she would visit us, at however unreasonable hour, i would offer her for tea etc to make her feel welcome. That kept me in her good books until now I guess. Now with LO with us, DH still goes to spend time with her and I think she wants the LO in there as well, but without me ????
After LO was born, I did ask her if she would like to spend some time with LO every week. She didn't seem interested so I can only assume that's because she doesn't want me there.
So you see her visits are not a problem, but her trying everything to make sure its when I'm not there is what's bugging me. Its been 6 months (24 weeks or 48 visits at least) and except once or twice, all visit have been when I'm away or supposed to be.

If my parents try anything like that, I would ask them not to and I know they will stop. But then I have always been very frank with my parents whereas I can see DH always puts MIL's feelings before common sense.
Also DH acknowledges that when they were kids, MiL used to hate her own MIL and never leave them alone with her.

Whilst I have been very careful about saying anything ever to make her feel bad (may be DH did later), she is not that kind to me. I joke about this often but i tell her how great she is looking and she replies by saying how badly dressed i am lol

after LO was born, she popped in one day and everything was sparkling except a table I had forgotten to dust which she decided to point out. I didn't mind and soon forgot about it. However a few days later SiL said something about the state of her house and dogs (MiL smokes inside the house and her dogs poop inside as well), and my MIL and DH were saying what a nasty person she is lol
I guess in future, I can try not to tell DH about my plans in advance and or may be go out when MIL is out walking her dogs. Or should I try to chat with her directly over this? I fear that might make things worse.

If DH had taken care of the hygiene issues then I probably this wouldn't have bothered me that much. Common sense does not work when it comes to his mum. E.g., DH is very health conscious, has a strong view against smoking but ignores if MIL smokes in front of baby.
I can certainly do other things like hiding all the mops, baby bottles, cutlery and even the dishwashing liquid etc before going away lol
DH likes a good chat, and both of us are very open. So even if a friend calls, we tell them everything. I dont think he calls MIL to tell her that I away, just mentions that like everything else.
I am thinking about what sistersofmercy101 said though, and do fear MILs negative opinion about women transferring to my son when he is older. Reason being when I met DH 12 years back, I had to spend some time convincing him not all woman are scheming bitches like his mum said. Its sad but my MILs poor opinion about women extends even to her 2 year old granddaughter (my SIL's daughter) whom she calls a fxxking cxxt as she cries when she sees her.
Having said that, after reading your replies, i can think a bit clearly now. I agree at the moment the issue is mostly around hygiene and if that's resolved I'll be less worried about leaving baby alone. So I'll have to figure out ways to do that without relying on DH.

Thank you for listening and helping :-)

I agree that she is trying to block me out.
I did think about taking the baby out but we live outside london and have to drive. LO hates sitting in the car. Will definitely do so when he is a bit older /cooler.
I agree DH has made the problem worse but MIL has always been the jealous type. If i buy a cake for DH on his birthday, she will ensure that for the next 5 years she bakes the cake herself for him instead. I don't like to get into such petty issues so keep on ignoring.
With my parents, if I don't like anything, I simply tell them. When my mum visited and forgot to change LOs nappy, i simply asked her to be careful and that was the end of it. Not sure why DH cannot. May be because she is widow, her elder son has moved 60 miles away with his family to avoid her and now she has no one else.
To be honest MIL has a generous side sometimes, like helps with DIY etc, and but the hygiene issues and now the idea that she is trying to block me out are a problem. Since SIL has moved away with her family, my brother in law visits her once a month with the kids alone. Perhaps she likes this arrangement of not ever having to accept another female in her family and is now subconsciously trying to do the same to me.
I spoke about the hygiene and health & safety issues with DH this morning, and he says he has spoken to MIL about it since. Lets see what happens next time she visits. Perhaps now MIL will try to avoid me even more lol