In the first 18 months of his life, Fran Newark's son cost her roughly $20,000. Newark, writing on Reddit under the name franny61390, was inspired by an earlier Reddit post where a user tallied how much money they spent on their dog in their first year of ownership, and decided to do the same for her kid.

What was expensive

Coming in at $9,900, the largest expense Newark lists is child care for after she and her husband returned to work. Newark and her husband pay a stay-at-home mom to watch him for $150 per week. Before hiring help, one of the highest costs for Newark's family came from their taking time off of work. She estimated the costs of lost income over the 14 weeks she stayed home with her infant to be $5,500. She took two weeks of paid leave and accessed short term disability pay via her employer for five weeks, and then took the last seven weeks unpaid. Newark estimates the cost from her husband's time away from work to be $2,346. He used some paid time off, so overall he lost about a month of income, according to her calculations. "The hit we took for taking time off was more than we thought it was going to be, but we were happy with that choice in the end," she tells CNBC.

Although Newark includes parental leave as a cost, she explains that it is more accurate to think of it as lost income, which she and her husband prepared for ahead of time by saving. "This was not something we had to pay, but something we had to account for in our budget," she writes. "We live below our means, so we were more than prepared to go without some income for a short period of time." Currently only about 13 percent of American parents have access to , as the United States is the only industrialized nation in the world that doesn't guarantee its citizens any subsidized time off to take care of family responsibilities. Recent surveys show that Americans generally support paid leave as a policy, but they disagree on who should pay for it and how.

What was cheap

One surprisingly low cost for Newark was health care. Her husband is in the National Guard, and she writes that his service comes with "amazing health care." For Newark, the cost of all prenatal, labor, deliver and postnatal care was covered with a single $25 co-payment through Tricare, and the family's monthly insurance costs did not increase after they added a child to the plan. She's fortunate as, , "the average national cost for a routine vaginal delivery is $8,775. The national average for c-sections is $11,525." The average American on health care.

How to save money on kids

Newark's No. 1 tip for saving money after having a baby is to be willing to buy or borrow used items. She says she was able to use social media to find deals on used products, and even found her child care provider through Facebook. They also borrowed items from friends who had babies. Though they haven't invested in toys, she notes, "he has survived happily!" "Focus on the things that you actually need instead of things you want," she says. "Which can be hard, especially if you are excited about having a new baby."

How they're handling finances going forward