In this, Baby Center focuses most of the content on strengthening your belly and back before getting pregnant.
However, it quotes and advice from Registered Nurse Julie Tupler, also co-author of Lose Your Mummy Tummy (Da Capo Press, 2004) that discusses toning your transversus abdominus, whether you're pregnant or not.
Transversus abdominus is made up of your deep abdominal muscles also referred to as core.
Tupler is an expert in Diastasis Recti Treatment with the Tupler Technique.
Her advice emphasizes "drawing in your belly button in toward the spine and holding it there while doing muscle strengthening exercises."
As per Healthline, the bottom line is "it’s safe to continue exercising if your pregnancy is normal."
If you want to dig deeper on that IF, do refer the American College of Obstetricians and Gynecologists.
The college has answers to list of exercise-during- pregnancy-related FAQs. See: FAQs
The WebMD cautions you to avoid specific exercises and activities during pregnancy.
Under "What Exercises Should Be Avoided During Pregnancy?", WebMD alerts DO NOT:
hold your breath during any activity, attempt skiing or horse riding as they involve risk of falling; contact sports including football and basketball as well as softball, volleyball that involve jumping; any exercise that's likely to cause abdominal trauma; Hopping, skipping, bouncing and deep knee bends, lying on your back for more than three minutes, exercise in hot and humid weather.