When I joined the military in 1991, I had to sign a form to say that if I got pg or in some circs, married (!) then I would be required to leave. And this (the pg bit at least) happened fairly routinely. It was one of the reasons that I initially signed on only for 6 years.
This was challenged via European Court of Human Rights, I think. The MOD had to pay an abso-freaking-lutely huge amount of compensation to women to had been forced to leave on pg, and to those who had had abortions so that they could continue to serve. Anyone reading the papers in the nineties would have seen the headlines as each settlement figure was announced.
I absolutely agree with the compensation payouts, but for those serving at the time who abided by the rules and put their personal lives on hold for their country, it was a bit of a pisser.
Anyways, so, big fat no. Pg servicewomen, whether 'accidentally' pg or not, are not required to leave. It costs the MOD too much in compensation. Amazing what effect a blow to the wallet has.
In fact, as a reservist, if they mobilize you for operations, the MOD will pay for your childcare if it was put in place to allow you to serve. 
I was doing load checks on hercs until I couldn't fit down the side in between the load and the airframe. 