gender reveal

/JH EH1 N D ER0 R IH0 V IY0 L/
noun
  1. 1

    An event at which the gender of an expected baby is revealed to family and friends as being either male or female (often by means of a cake, balloons, etc., whose blue or pink colour indicates the gender).

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Source: https://en.wiktionary.org/wiki/gender%20reveal