pressure from work

I am a school social worker and have summers off. my boss just informed me that there is a training opportunity she wants me to attend july 11-14 that is from 8am-4pm daily. the training would be me sitting all day long, little movement. i also have a 40 min commute to the training site one way making for a long day. i am due july 19th and would rather spend these last weeks prepping for baby. not to mention sitting for long periods aggravates my tail bone and sciatic nerve. i am feeling a lot of pressure from work to attend this and am afraid it will be held against me if i do not attend when i return from maternity leave. they wouldnt even be paying me because it is considered an "optional" training. anyone hve any advice for me or been through something similar?