Build expression to calculate weeks of gestation in a pregnancy

[mkappess]

I need to build an expresssion to identify weeks and days of the duration of a pregnancy. If due date is = to 40 weeks, then I need an expression to identify due date ( 40 weeks identified by a specific date) minus delivery date to yield the gestational age of the newborn. Any ideas?
I'm self taught in Access, please keep it simple. and if you can write out the expression for me that would be awesome!

[ranman256]

a form with text box of start date
a box for due date. txtDue = dateadd("d",280,txtStart )
a box for gest. calc txtGes = datediff("d", txtStart,txtDue)
(d is for days, so divide by 7)

[mkappess]

a form with text box of start date
a box for due date. txtDue = dateadd("d",280,txtStart )
a box for gest. calc txtGes = datediff("d", txtStart,txtDue)
(d is for days, so divide by 7)

Thanks for replying.. .. since I don't know the actual start date of the pregnancy, I need to build an expression using the due date, (the due date= duedate - 280 (would be the approximate pregnancy start date). then I can use the rest of your expression. However, when the report is made, my boss is looking for a 'weeks + days" format. For example the gestation was 37+4 (37 weeks +4 days).Thats how they determine if the baby is premature, and we are tracking premature births. Do I just have to do that manually, or can Access do a weeks + days total?
Thanks

[ranman256]

You'd need a routine to break up the days/weeks...

Public Function ElapsedTime(ByVal pvDate1) As String
Dim iMos As Integer, iDays As Integer, iHrs As Integer, iYrs As Integer, iWeeks As Integer
Dim vElaps
On Error GoTo errElaps
Dim vNow As Date
vNow = Now()
iDays = Abs(DateDiff("d", pvDate1, vNow))
  If iDays > 365 Then
       iYrs = iDays / 365
       iDays = iDays - Int(iYrs * 365)
       
       vElaps = iYrs & " years "
   End If
   
  If iDays > 30 Then
       iMos = iDays \ 30
       iDays = iDays - Int(iMos * 30)
       
       vElaps = vElaps & iMos & " Months "
   End If
   
  If iDays > 7 Then
       iWeeks = iDays \ 7
       iDays = iDays - Int(iWeeks * 7)
       
       vElaps = vElaps & iWeeks & " weeks "
   End If
   
  
  If iDays >= 1 Then vElaps = vElaps & iDays & " days "
    'hours here
    
errElaps:
ElapsedTime = vElaps
End Function