What does a right to family planning mean?

For more than half a century, advocates have asserted that couples have a human right to family planning— the knowledge and means to determine the timing and spacing of their pregnancies and births.  Yet in practice, nearly half of all pregnancies are classified as unintended, despite near-universal knowledge of and access to family planning methods.

How did the idea of a right to family planning emerge in the first place, and how is such a right realized in light of human behavior and biology, without coercive measures that would infringe on other rights?

When nations adopted the Sustainable Development Goals in 2017, and pledged to spend billions implementing them, this right was included. However, confusion about what this right is, and how it should be recognized, persists decades after nations first considered it a priority. Translating the aspirational language of UN resolutions about family planning into scientifically measurable terms has been difficult, if not impossible. That is because of a much more fundamental confusion about the nature of the right and how it was established.