Is it permissible to perform tayammum for wudu or ghusl while being able to wash some limbs, and to suffice with wiping over the feet to avoid the strain of pregnancy, and am I sinful if the fetus is harmed due to insisting on wudu or ghusl?

Legal commands are based on ability; what cannot be done is dropped, and what can be done is performed. Therefore, washing the feet is not obligatory if there is difficulty; it suffices to pour water over them or immerse the feet in a basin of water. It is permissible to seek help from family members in case of inability. If you are unable to do all of that, then tayammum (definition) (dry ablution) is prescribed. Exercising a concession when there is a need is permissible and beloved by Allah, and it is not permissible to harm oneself or the fetus.