Hi Rhiannon,
Congratulations on your second baby. How old is your first? As for your question...... Each pregnancy is different, even in the same women. Even though you stayed with your pre-pregnancy exercise program throughout your first pregnancy, you may have to modify this time around. Second pregnancies tend to be different from first ones. All the changes seem to occur sooner and some of these changes and symptoms may impact your ability to exercise at your pre-pregnancy level.
I think you would gain a vast amount of knowledge by reading throught the excellent questions that have been posted on our forum. I also suggest that you buy "Exercising Through Your Pregnancy" by Dr. James Clapp III. It is an excellent book for the active woman regarding exercising through the childbearing year.
In response to your heartrate question.... The 140 heartrate guideline is obsolete (since February, 1994) I suggest that you go to the post "Question on safe heartrate for pregnancy" posted on our forum by CandiceO on 10/3/99 for a discussion on the topic.
Also, there is an excellent post on weight training posted by Aivilo on 7/20/99.
Hope this helps. Let us hear from you.
Sheila