Nor do your choices have to be restricted to one arrangement. “Sometimes they go to day care three days and have a nanny for two days,” Ms. Harad said. “There is no black or white answer but a whole array of gray options.”

WHAT OPTIONS COST Some parents believe that the group dynamics of day care will help prepare their child for the world, or at least kindergarten, while others are willing to pay top dollar for a responsive nanny who won’t be distracted by other children. But for many families, the arrangement will largely be driven by what they can afford.

In most child care situations, costs tend to vary by the child’s age, the hourly commitment each week and where you live. According to Child Care Aware’s research, in the District of Columbia, the average cost of full-time day care for an infant was $21,948 annually. In Massachusetts, it was $16,430 a year, and in New York, $14,939.

Nannies typically cost more, though they can often become more cost-effective when you have more than one child. According to Care.com, the national average hourly rate for nannies is $11.73 and ranges from $8.26 to $15.19 an hour, depending on the state; those figures reflect the rates posted on a sampling of the 150,000 job postings on their website for roughly the last year. The nanny pay scale rises in expensive metropolitan areas, of course, including New York. Parents who employ nannies in brownstone Brooklyn, for instance, pay anywhere from about $12.75 to $19.25 an hour, according to Park Slope Parents, a local parents group.

When you consider those numbers, au pairs start to seem like a relative bargain, as long as you have an extra bedroom, provide meals and don’t mind the live-in arrangement. Au pairs — who are 18 to 26 years old and in the United States on a special cultural exchange visa regulated by the State Department — generally live with a family for a year, but they can stay up to two years. Cultural Care Au Pair said it generally cost $355 a week (less than $8 an hour for a 45-hour week, or about $18,000 annually) to hire an au pair, but that includes items like their medical and travel insurance. Another agency, Au Pair in America, had similar pricing.