Access:Average Year to Date in Query

I have a table(Name: "Deliveries", Columns: "Ontime", "Late", "Month", "Year"
When a delivery is ontime it is labled 1, when it is late it is labled 0

The SQL below lists each month, number of deliveries ontime, number deliveries late, as well as the number of average deliveries on-time for each month. How do I add Average year to date to the mix?

Current CODE:
SELECT Deliveries.MonthandYear, Sum(Deliveries.ontime) AS [Delivery On-Time], Format(Avg([ontime]*100),"Fixed") AS [Percent Delivery On-Time for month], Sum(Deliveries.late) AS [Late Deliveries]
FROM Deliveries
GROUP BY Deliveries.MonthandYear, Deliveries.year_thru, Deliveries.month_thru
ORDER BY Deliveries.year_thru, Deliveries.month_thru;

Current Table Looks Like:

Field:              Month                Delivery On-Time   Delivery Late   Percent Delivery On-Time for month
Row1         August 2006                     53                      20                                   72.60            
Row2         September 2006              30                       10                                   75.00
Row3         October 2006                   60                       5                                     92.31
Row4         November 2006               20                       3                                     86.96
Row5         December 2006               15                       1                                     93.75

I NEED THE TABLE TO LOOK LIKE THIS:

Field:              Month    Delivery On-Time   Delivery Late   Percent Delivery On-Time for month      YearToDate
Row1       August 2006           53                      20                                  72.60                                     72.60      
Row2       September 2006     30                       10                                  75.00                                    73.45
Row3         October 2006        60                       5                                    92.31                                   80.34
Row4         November 2006    20                       3                                     86.96                                   81.09
Row5         December 2006    15                       1                                     93.75