I have 3 boys. DS1 was a severe SD 15y ago. Nearly died, apgar score of 1, came out dark blue. Special care. Very scary. No known side effects, except he struggles at school a bit and I often wonder if that's related as he was 8 minutes before resuscitation according to my notes.
Obviously we were extremely worried for DS2. We insisted on having a resuscitator in the delivery room and they had a second senior midwife assigned. Of course they wouldn't believe me when I said he was coming and both went on a tea break at the same time
and DS2 shot out like a rocket, caught by my DH who pressed the red button with his foot. But no SD at all.
DS3 was a third vaginal birth. He did get stuck and I had a second SD, but it was less serious purely because I had a better midwife supervising a student midwife and he leapt in, shoved his massive hands up and managed to unwedge DS3 who was stuck behind my pelvic bone again and he came out relatively easily after that (stitches for me ..)
So my experience is 3 boys, 3 vaginal births, 2 shoulder dystocias, 8lb 4, 8lb 2, 7lb 11, so not particularly massive for boys. However I am short (slightly under 5ft) and DH is 6ft, which I think predisposes you to a SD. More so though if you look at the 3 boys now as 15,13,10 year olds, you can totally see how/why it happened. DS1 and DS3, who did get stuck are short and stocky, rugby player build with massive heads. You can't see this to look at but they've always had wide head circumferences, broad foreheads, always had adult men hat's etc and their head circs were always measuring up in the 90th percentile during pregnancy, so I should have known. DS2, who didn't get stuck, has a totally different build - taller, much slimmer, narrower shoulders and a completely different shape head, longer and thinner rather than round and slaphead
.
Not that this really helps beforehand but is interesting to me to see why the stockier ones got stuck. If your second baby is looking chunky with larger head circumference, imo, it might be more likely it will happen a second time, though of course, it might not.