Access BeforeUpdate event fires too soon

I am inputting a new record on a screen whose data source is just one table. There are in the VB some Recordset references to other tables, but the referring control on the screen is unbound. When I have input 6 items, starting with the index and only required item, control enters the Form_BeforeUpdate procedure, although there are further controls on the screen for the record. It does not matter whether tabbing is to one of the further controls, to a combo box on another table(using the index from this screen) or to a subform contining two controls for this table.
How do I arrange that the BeforeUpdate event be triggered only when the user has entered all the items, or at least passed through them?