Gestational sac size and week number comparison table (first trimester embryo development guide) Gestational sac size and week number comparison table is something that pregnant women are very concerned about. Knowing the size of the gestational sac can provide a rough idea of the embryonic development. The following is a detailed introduction to the comparison table of gestational sac size and weeks from at least four aspects in the article. Part One: The development process and size of the gestational sac. The gestational sac is the initial stage of formation after fertilization of the egg. As the embryo develops, the gestational sac is constantly changing. Under normal circumstances, the gestational sac will appear on the 5th to 6th day after fertilization, with a diameter of approximately 0.1-0.2 mm. Around the 10th day, the diameter of the gestational sac is about 2 mm, and the fetus can be seen around the 21st day.