I don't want to have a baby?

I've been with my girlfriend for 5 years now and she wants to have a baby and she has been considering all different options including IVF and self insemination. I do not want to have one and i do NOT want to have to give birth, but i've said yes to it. I feel obligated to do it because she has taken care of me for so long. When i told my mum she said i was being selfish and i should just do it since i love her anyway, a child would just make the relationship stronger. Is it apprehension of being a first time parent, the complicated procedure, pregnancy and child birth, or am i being selfish?