Baby fruit size? Is the baby like a lemon or an apple? Here's a baby and fruit size chart to help you learn how your baby is growing and the corresponding fruit. In just 40 short weeks, your baby will grow from the size of a tiny seed to the size of a plump watermelon. 2 of 39 Your baby is almost three inches long now, about the size of a passion fruit. Fingernails, toenails .. Your Pregnancy To-Do List. 4 weeks: Baby is the size of a poppy seed. Your pregnancy may be detectable in a week or so by a home pregnancy test. See a detailed image of a baby in the.