My last baby had a knot in the umbilical cord but did OK. I just lost a baby at 20 weeks with a tightly twisted cord. Did I do anything wrong, or can I do anything to prevent this next time?
Cord accidents, as they're often called, are when mechanical conditions of the umbilical cord cause an obstruction of oxygen to the baby. A tight knot in the cord can do this, but thankfully most true knots are incidental findings at the time of the delivery with no perceived harm. Only one out of 100 deliveries involve true knots, and out of these only one in 20 have a life-threatening constriction. The arithmetic on this means only one in 2,000 deliveries will manifest this complication.
Thankfully, a substance called Wharton's Jelly provides a cushioning effect around the important blood vessels of the cord and protects them even with significant coiling and accidental knoting. This no doubt accounts for the small number of problems.
Most of the time.
Of course, there are the exceptions, and certainly yours is one of them. Conditions that make a woman more prone to suffer this type of tragedy are those in which nutritional deficiencies affect the structure of the cord, specifically, Wharton's Jelly. Smoking, hypertension, drug use, and other factors will increase the likelihood.
A short cord (usual length is about 55 cm) will mean less slack should the cord wrap once around the neck. This will tighten the remaining cord, especially as the baby descends during labor and delivery. A loop around the neck happens a lot—one in five, but thanks to adequate length and Wharton's Jelly, is seldom a problem.
Two loops and more occur in longer cords, but in normal length cords can pose a significant tightening phenomenon, not so much around the neck but of the cord itself. This will be seen as abnormal fetal heart rate on the fetal monitor during labor. (One out of a thousand will have three or more loops around the neck.)
Your problem seems to be coincidental, and I would venture to guess that someone with your history has no greater chance of this recurring as anyone.