NOTHING can sink a friendship like differences over parenting. Sometimes the areas of disagreement are stark and dramatic, leading to blowups and out-and-out breaks. Most of the time they’re subtle and unstated, a matter of dark looks and long-simmering resentments, that erode, rather than rupture, formerly close relationships. Often they arise from a vague sense of betrayal, a friend’s having changed once he or she has had children, breaking unspoken assumptions about shared values and goals, how to live and who to be.

It’s the sort of relationship-fraying challenge portrayed with much humor in the film “Friends With Kids.” And it’s one that a Washington mother of three found herself forced to confront when a close friend became pregnant, revealing an entirely new side of her personality. “She immediately stopped her temp work because the Xerox machine might be bad for the baby,” said the mother, who, like several others interviewed for this article, requested anonymity so as to not compromise her relationship with the friend. “She changed all her shampoos. We pretty much had to detox the environment whenever we saw her from then on.”

The tensions deepened, she recalled, once the baby was born: “She practiced total attachment parenting. She never let anyone watch her baby. To go to a movie, she and her husband would go one after the other. If it was cold out, she’d bring the car seat into the house and warm it with a blow dryer” before bringing it back to the car. When the child was older, she said, “you weren’t allowed to say no to him. You weren’t allowed to set boundaries. We were at our wits’ end.”

No matter the cause, no matter how well-managed the reaction, the disagreements arising over parenting practices can hit hard and cut deep. Because what’s at stake is much more than different ideas about Ferber versus Sears, or organic versus conventional, or the use of timeouts, or the limits to be put on TV time. What is often triggered, in the divide between what mothers and fathers do or don’t do — whether or not those differences escalate into out-and-out confrontations — are convictions that push all the most basic parent-buttons.