If I understand things right (big IF) stethoscopes do not amplify sound, only conduct. So it would be the same as holding the egg up to your ear.
Correction, they do amplify a little via the diapragm in the drum of the stethoscope. The surface of the the working end makes poor contact with the egg and all you can hear is a scratching sound of the shell on the scope surface. If you made a soft adapter to fit over the end of the scope to cushio the egg, it might work. I only know this becuase I have a stethoscope, and i just went to the incy and tried it on a 17 day developing egg.
I've got an incubator full of pekin duck eggs. We purchased the incubator last spring & had an excellent hatch rate.
1. I only have 2 female ducks, so I put 4 eggs in the incubator on Day 1 & set the incubator settings & timer for 28 days. I followed instructions from Cornell Universitys website.
2. I put 2 eggs in the incubator each day after that until the incubator was 3/4 full.
I was quick about it, as not to disturb the incubator settings & added water as appropriate to keep setting correct.
3. The 28 day egg timer expired almost a week ago! The first 4 eggs should have pipped by now but...nothing!
4. I've candled them a few times since then to check for signs of life.
a. 17 eggs eggs are healthy & developing. I can see healthy blood vessels & movement.
b. 2 eggs are duds (not fertilized).
c. 5 eggs started to develop but likely died. I can tell because the blood vessels appear to be breaking down into clots & air sac is no longer visible.
d. I can't tell if the 4 eggs that should have hatched already are alive or dead. The air bubble is the appropriate size for this stage of development but I can't hear or see the ducklings moving inside.
Its tough to see much of anything during the final days cuz the duckling fills up most of the egg; but last year we could see the ducklings little face poking up through the air bubble inside just before it pipped & we could hear it making noise as it tapped on the shell. I don't see or hear that this year with the overdue eggs.
I've attempted to listen to the overdue eggs with a stethoscope. The small side of the stethoscope works with a steady hand, however, I may just be hearing my own body sounds through the stethoscope because my hand is holding the egg...so I put the egg down on a secure surface & listened for signs of life again but didn't hear anything.
Muscovy ducks can take 35 days to hatch but Pekin ducks should take 28 days.
I don't want to remove the overdue eggs in case they are alive & soon ready to be born, but I worry about keeping them in the incubator if they are dead because a couple dead eggs exploded in our incubator last year & the smell & mess was so horrific, that it still lingers a year later, even after hosing & sanitizing the incubator last summer.
We used an old incubator last year, before purchasing the new one that is currently in use, which could hardly maintain a consistent temp/humidity setting, but we still had a good hatch rate, so I don't think opening the incubator for a few seconds each day to add 2 additional eggs caused a problem, especially since most of the eggs are healthy & developing.
The biggest problem we had with the old incubator was "sticky chick", which was quite sad. The fluctuating humidity level made it difficult for several healthy babies to hatch & they required assistance. A few died because I assisted before they'd fully absorbed the yolk sac. One died because I failed to assist. Another suffocated to death in it's shell while I was helping. That was terrible! I used dental floss to tie off the sacs, a blow-dryer...even did CPR on a few. It was very sad but we still had many healthy babies born. I didn't want to risk sticky chick happening again so we got the new incubator.
Anyway, I guess I just have a little bit of PTSD now. I don't want babies to die trapped in their eggs. But I don't want to intervene unless absolutely necessary. And I definitely don't want rotten eggs exploding all over our healthy developing eggs because it's impossible to clean without removing the developing eggs & hosing out the incubator.