A twin gender ultrasound at 21 weeks is generally reliable, but not always 100% accurate. The accuracy depends on various factors such as the position of the babies, the experience of the sonographer, and the clarity of the images obtained. In some cases, one baby may be easier to identify than the other due to their positions or movements. Additionally, if one baby is blocking the view of the other or if they are positioned closely together, it may be more challenging to determine their genders accurately. It's important to remember that ultrasounds are not infallible and there is always a small margin for error. If you have any doubts or concerns about your twins' gender determination, it's best to consult with your healthcare provider for further clarification or consider additional diagnostic tests like amniocentesis or genetic testing for confirmation.