You might have a blighted ovum. When an egg is fertilised by a sperm, some cells develop into the
embryo, and others develop into the placenta and membranes.
Sometimes, the cells that would have become the embryo fail. This is probably due to there being too many or too few chromosomes during fertilisation. But the cells that become the placenta and membranes keep developing, so the pregnancy sac continues to grow, and the placenta starts to form.