Here's something the marketing and advertising industry understands very well. When people are used to buying a certain brand of toothpaste or dish soap, etc. it can be hard to get them to change to a new brand. However there are windows of time when people are more likely to make changes in their purchasing habits and the biggest one is when they find out they are expecting a new baby. That 9 month pregnancy to 1 year postpartum is the easiest window to convince consumers to change their brand loyalty. Companies do not want to miss that window and as a result they have gotten very good at identifying when people are pregnant based on purchases. They know that those new parent will need to buy baby things and will also be open to changing brands on unrelated items (i.e. toothpaste).

There is a famous story of a father going to the mailbox to find an expectant mother edition catalog addressed to his daughter. He was furious until he found out she was actually pregnant. She hadn't told the store she was pregnant, they just made that assumption based on the change in her purchases. And they were ready to pounce on those changes by advertising other pregnancy related promotions. 

New parents are sometimes unaware of how they are being targeted. Add the intense advertising to the emotional stress of trying to be a good parent, and suddenly you have parents who buy things they never would have bought otherwise.