As a general rule in the State of Washington, child support is not retroactive prior to the date of the filing of the petition asking for child support. However, there are a few circumstances where child support can go further back.
One circumstance is where the parents were not married. In a Paternity action, the court can order a person determined to be a parent to contribute to the birth expenses and can order retroactive child support back five years. I believe that the theory behind this is that while there is a presumption that a married couple has been covering the child’s expenses jointly while they were still together, it is assumed that a single person who is later determined to be a parent may not have been contributing to the child’s expenses up to that point. I would expect that a court would take into consideration any support that had been voluntarily paid previously (so make sure you can document any voluntary support paid), or any sharing of expenses while the couple lived together.
There can sometimes also be a retroactive modification of child support IF the current child support order calls for support to be modified as of a specific date and that date has passed. While child support usually cannot be modified retroactively prior to the modification/adjustment action, it can modify the support back to the date that the prior court order stated that support should be modified. To avoid this problem, it is a good idea to not include mandatory modification language in support orders, but rather to provide provisions as to when support CAN be modified.