At what gestational age does the amnion typically adhere to and fuse with the uterine sidewall (decidua)?

Amnion-Chorion Fusion Timing

The amnion typically fuses with the chorion (which is apposed to the uterine decidua) between 14 to 16 weeks of gestation, though this process is generally complete by approximately 16-17 weeks. 1, 2

Normal Developmental Timeline

Early First Trimester (5-7 weeks)

• The amnion first becomes visible on ultrasound at approximately 7 weeks gestational age as a thin membrane surrounding the embryo within the gestational sac 3
• At this early stage, the amnion is clearly separated from the chorion, with the extra-embryonic coelom (chorionic cavity) visible between them 4, 2
• The amniotic cavity is initially small and distinct from the larger chorionic cavity 4

Mid-First Trimester (8-13 weeks)

• Separation of the amnion from the chorion before 13 weeks is completely normal and should not raise concern 2
• The amnion can be consistently visualized in approximately 84% of pregnancies before 12 weeks 4
• During this period, the amniotic cavity progressively enlarges while the extra-embryonic coelom (space between amnion and chorion) gradually diminishes 1, 2

Late First to Early Second Trimester (14-17 weeks)

• The rapid growth of the amniotic cavity leads to obliteration of the chorionic cavity and the amnion progressively approaches and fuses with the chorion 1
• By 16-17 weeks, fusion is typically complete, with the amnion now adherent to the chorion, which itself is apposed to the maternal decidua 1, 2

Clinical Significance

Normal vs. Abnormal Separation

• Before 13 weeks: Amnion-chorion separation is physiologic and expected 2
• After 17 weeks: Persistent separation of the amnion from the chorion may be abnormal and warrants further evaluation 2
• The 13-17 week period represents the transition zone where fusion normally occurs 1, 2

Important Distinction

• Separation of the chorionic membrane from the underlying decidua is abnormal at any stage of pregnancy and differs from normal amnion-chorion separation 2

Practical Ultrasound Implications

• When performing first-trimester ultrasound, visualization of the thin amniotic membrane separate from the gestational sac wall is normal and expected 3, 4
• The "double bleb sign" (amniotic sac and yolk sac) can be identified as early as 5-6 weeks, demonstrating the distinct amniotic cavity 2, 5
• Clinicians should not misinterpret normal first-trimester amnion-chorion separation as pathologic 2