Annie, did a search and found the info. on exercise modifications during pregnancy that Cathe recommends. I guess she doesn't specifically mention doing push ups against the stairs but does suggest doing them against the wall.
Here's Cathe's prenatal recommendations for exercise:
During the first trimester the main change in your workouts is that you do not get your heart rate up super high (gasping for breath), which will also remain one of your priorities for the duration of your pregnancy. Sheila, our prenatal expert, and myself, who has been certified by Sheila, recommend that you use the talk test and/or perceived exertion test rather than a heart rate monitor or chart because your heart rate is already naturally elevated when you are pregnant therefore heart rate charts and monitors providing you with a false indication of your actual intensity. Your goal should be to say 2 to 5 word sentences at all times during your workout. How many days you workout will vary on your energy level. While you are pregnant, especially in your first trimester, you may feel nauseous, tired, and unmotivated all contributing to just how many days you actually do and/or how hard you workout per day. But the overall concern here is to listen to your body day by day and only do what your body is telling you it can do each day. If it says "no workout today" then take the day off. You may have a spell where you need a week off, you just never know.
Also, impact should be reduced as you get further along because your pelvic floor will be stressed and ligaments in the pelvic area extremely stretched to support the weight of the baby. This is the time to point out how important it is for you to do kegals constantly, anytime, anywhere! Intricate choreography should be done with extreme caution (or even eliminated)as you get into the further stages of your pregnancy since your center of gravity will change causing you to potentially lose your balance.
SIDE NOTE: I should also point out that ACOG has revised their guidelines in 1994 stating that is is now okay for a woman to get their heart rate above 140 while working out. They too recommend using a perceived exertion test to monitor your intensity level. I mention this to you since you will still find many people unaware of this change.
As for your weight training, you should be fine continuing with this until it just naturally becomes uncomfortable for you. You certainly can use heavier than 5 pounds if this is what you have normally done. The main concern is to not be flat on your back after 12 weeks and to be aware that a hormone relaxin in now in your body. Relaxin naturally softens the bones, ligaments, and joints to prepare your body for child birth. Many experience the effects of this much more than others. In any case, as long as this hormone is present in your body, you should not be pushing your self to do extreme weight because your support system is not what is usually is. I kept doing my usual weight until suddenly one day I started to just feel that my usual weight was a struggle and/or I would feel unusual pulls or clicking sounds in my body while doing a certain exercise. All flat back exercises such as bench press, crunches, etc. should be done on an incline after 12 weeks as to not reduce the circulation that goes to the baby. Other exercises should always be done with caution and your body should always be supported. For instance if you are leaning over to do tricep kickbacks, have one arm (and possibly even your leg too) up on a bench to support your weight and give you back support. Exercises such as planks, leg raises, and push ups should be done at your discretion. I would STRONGLY suggest the modified bent knee versions of planks and push ups because they are very demanding poses to your lower back. Eventually when your belly gets very big you yourself will come to the realization that these exercises are simply not doable anymore. At that point, eliminate planks, if you haven't already, and do your ab work on all fours where you will have the baby weight to use as your resistance. Push ups can be done against the wall at this point. Long lever moves such as leg raises will most likely just be too uncomfortable due to balance issues and your pelvic area being very sensitive at this time. Remember, the relaxin in your body is softening your joints therefore weakening your support system to some degree.
As for your ab crunches, as I mentioned you can do it on an incline even though it IS a bit easier this way (but safer for baby). The all fours position will not do very much at this point but when your belly gets very big, you will have a nice "ball" to use for resistance.
Stacey
Baby#3 due 1/22/07
Emma born 4/27/03
Alex born 9/7/01
And on Thursday I finally started to feel her moving. How awesome, what an amazing feeling!!! :7 Now I can never wait to feel her move again. ;-) 
