Need to create a variable that counts the number of previous events by ID

[mdwilson]

I have a large data set with multiple births per mother. I'm studying the occurrence of an adverse event for the infant. I was to know if the number of infants that had the adverse event previously for a given mother affects the chance of the adverse event happening for subsequent births. The first birth for each mother should always have 0 previous events, but I need to include sum the number of events after that, including if the event happens on the first birth. 

So I have:

data have;
input mom_id event ;
datalines;
1 0
1 1
1 0
2 1
2 1
2 0
3 0
3 0
3 0
4 1
4 1
4 1
;
run;

These are the data I want:

data want;
input mom_id event prev_events;
datalines;
1 0 0
1 1 0
1 0 1
2 1 0
2 1 1
2 0 2
3 0 0
3 0 0
3 0 0
4 1 0
4 1 1
4 1 2

;

run;

I have a lot of experience with SAS, but not with macros or loops. Thanks for your help!

[PeterClemmensen]

I think this is what you want

data have;
input mom_id event ;
datalines;
1 0
1 1
1 0
2 1
2 1
2 0
3 0
3 0
3 0
4 1
4 1
4 1
;

data want;
   set have;
   by mom_id;
   if first.mom_id then prev_events = 0;
   prev_events + ifn(first.mom_id, ., lag(event));
run;

Result:

mom_id  event  prev_events
1       0      0
1       1      0
1       0      1
2       1      0
2       1      1
2       0      2
3       0      0
3       0      0
3       0      0
4       1      0
4       1      1
4       1      2