The placenta is first visible on US as a focal thickening along the periphery of the gestational sac from around 8 weeks of gestation. By the end of the first trimester it can be identified as a fine granular, disc-like structure occupying a major part of the endometrial surface. Cumferential growth occurs throughout most of pregnancy. As it matures the placenta develops a stronger echo pattern than the underlying myometrium. The chorionic plate is seen as a bright linear echo lying between the homogeneous echoes of the body of the placenta and amniotic fluid.

 

Gestational age (weeks)                                            Sonographic findings

          8                                                                       Focal thickening of the gestational

                                                                                   sac wall is first visible

         12                                                                     Placenta is clearly visible in most cases.   

                                                                                  Echogenicity is initially similar to the

                                                                                  myometrium but increases

        16                                                                      Placenta and fetus are of similar

                                                                                  size 

       20+                                                                    The placenta is usually 2—3 cm thick (max.                          

                                                                                  normal = 4 cm; placentas more than 5 cm

                                                                                  thick should be considered pathological)

 

      29                                                                        Placental calcification becomes visible

 

     33                                                                         50% of placentas now shoe foci of calcification