Middle shadows between twins? !?!

gill • I'm a mother of 5 beautiful babies, 4 girls chloe 10 lucy 4 Molly 3 and Macie 19 months, and my gorgeous son Adam 7 😄 I've been with my husband Cyril for near 9 years and married for 2 and a half xx
Can anyone tell me what thus black shadow/bubble is in between my twins? I've had a few ask me but I really don't no? I thought maybe a cyst? Anyone else who had twins have anything similar? Also in pic 2 it's above one if the twins x