Jesse comes back from 6 weeks on tour and Rebecca is much larger than when he left. She wouldn't have gotten that big in 6 weeks.
Jesse & Becky get married on Valentine's day. Becky finds out she is pregnant soon after Michelle/DJ's graduations (late May/early June). Around the month of September (return of school for the kids) she finds out about the twins. Michelle's birthday is in November same day as the birth of the twins. This would mean Becky found out her pregnancy at about 4 months and didn't find out about the twins until about 7 months along. Realistically a mother would find out way before then.