In your first month of pregnancy the embryo resembles a tadpole. The arms and legs begin as tiny buds. Nerve development begins and the backbone, spinal cord and lungs begin to form. The heart also begins to develop and will start beating near the end of this month. The placenta is present but not fully developed. The embryo will be approximately ½ inch long and will weigh a fraction of an ounce at the end of the first month.