First Trimester Baby Development. During this trimester, the fertilized egg will change from a small grouping of cells to a fetus that is starting to have a baby’s features. In these first few weeks of pregnancy, your little one’s brain, spinal cord, heart, and tiny limbs — complete with fingers and toes — will form.
The third month of the first trimester is considered the most crucial for all mothers out there. Your baby is growing and developing rapidly, and is most at risk from hazards such as smoking, alcohol, drugs, infections and x rays. He or she will even be practising swallowing!