Weightlifting is recommended to build muscles used for carrying the unborn child. Neither of these activities may be appropriate in the second or third trimester, however. For example, while walking is almost always beneficial, bicycling may not be. The dangers of falling off a bike pose big health risks in the late stages of pregnancy. Even a stationary exercycle could pose a hazard.
Nutritional needs vary over the course of the pregnancy as well.
Neither helps the baby or you. The need for calcium and protein is also critical. I don't really go in for the new high protein diets. But I do recognize that developing babies need adequate amounts of calcium to develop their bones, and more protein to develop muscle tissue. Mother also needs these nutrients as to develop the strength needed to carry and care for the baby.