Can a fetus be detected at 4 to 5 weeks on scan?

A fetus typically cannot be detected on a scan at 4 to 5 weeks. At 4 weeks of pregnancy, the embryo is still very small, about the size of a poppy seed. It is surrounded by a sac filled with fluid, called the amniotic sac. The embryo is located at the top of the uterus. At 5 weeks of pregnancy, the embryo has grown to about the size of a sesame seed. The embryo is starting to develop its major organs. It has a heart, brain, and spinal cord. The embryo is also starting to develop its limbs. However, at this stage of pregnancy, the embryo is still very small and difficult to detect on a scan. A transvaginal ultrasound, which uses a probe inserted into the vagina, may be able to detect the gestational sac and possibly a yolk sac at this stage.