Late to this thread, but you ought also to be aware of the surprising, often unwelcome, things that can happen to you when you've had children. I was one of those who thought I'd be finishing up a book on maternity leave (ho ho ho). I had been used to working the usual 60-70+ junior academic's working week, had done so for a while, and expected to be able to at least carry on in the same style, albeit doing a bit less.
I simply hadn't realised how much those working hours were enabled by being young, fit and well-rested. As pp have said, it's impossible to do any research, never mind maintain high working hours, when you haven't had any sleep. (Enough childcare is a whole other issue I won't get into. But you will need LOTS of money or supportive family living close by.)
From being someone who was fit and cheerful and rarely got ill (maybe a cold a year?) I had a bad birth, which left me unable to walk properly for two months and having physio for much longer. I had numerous other health problems following the birth, put on loads of weight, and my immune system absolutely crashed - by the time my DC was about a year old I was getting ill, and I mean having to go to bed for a couple of days or more ill, every week or two weeks. My doctors could not find any particular reason why this was happening: probably a combination of poor immune function, sleeplessness, stress, weight gain, and having a germy toddler. Some of it, my GPs thought, was probably down to a degree of physical burnout having worked such long hours for years beforehand - it's true that the high workload demanded in academia seems fine as long as you can keep it going, but you never know when you're going to hit a physical wall. (It is not healthy to work 70 hours a week for 10 years straight.) There was nothing I could do about getting ill like this and nothing I did seemed to make it better. My oldest DC is nearly 3 and I've been pretty much suffering from low-level illnesses almost constantly for 2 or more years. You name it: respiratory viruses, noro, throat problems, skin infections, mastitis, gum abcesses, conjunctivitis, more and more flus and colds, you name it, I get it.
Obviously not everyone will suffer from this; but it's pretty common to suddenly find you're getting a lot of minor and chronic illnesses and infections on a regular basis with small DC. I have (academic) friends who have suffered from a variety of problems post-DC. I have been able to do my teaching, and publish a couple of reasonable articles and reviews in the last 3 years (am in humanities, thankfully in a low-publishing field compared to others), but I'm aware that this is causing my research career to crash and burn. My manager is understanding, but I feel a lot of guilt.
So it is just not possible to predict everything that you will face after having DC. My expectation that I could bounce back and trust to my previous physical capability was not correct.
Get the PhD and the job before having the baby - you have loads of time!
If you want to feel less broody, try reading some of the forums and threads on here about post-birth complications. These aren't rare things - forceps, tears, physio, illness, babies that are ill or don't sleep - they happen to loads of women, especially first time mums. Being realistic when you think about what is involved in having a baby may make you reconsider for a while, at least! 