What is the most common factor for e coli infection in pregnancy?

Inoculation!: The same factors exist. Autoinoculation, or from sexual partner. However, smaller void problems, pressure on the bladder, make autocleansing worse.

Proximity: You didn't state UTI but E.coli is a common cause. Your poop is filled with E.coli and the proximity of your anus and makeup of your genital structures make cross contamination common in girls/women. Careful front to back wiping and voiding after sex can reduce risk.