BMI with links to Height and Weight

{
  "resourceType" : "Observation",
  "id" : "bmi-using-related",
  "status" : "final",
  "category" : [{
    "coding" : [{
      "system" : "http://terminology.hl7.org/CodeSystem/observation-category",
      "code" : "vital-signs",
      "display" : "Vital Signs"
    }],
    "text" : "Vital Signs"
  },
  {
    "coding" : [{
      "system" : "http://loinc.org",
      "code" : "39156-5",
      "display" : "Body mass index (BMI) [Ratio]"
    }]
  }],
  "code" : {
    "coding" : [{
      "system" : "http://loinc.org",
      "code" : "39156-5",
      "display" : "Body mass index (BMI) [Ratio]"
    }],
    "text" : "BMI"
  },
  "subject" : {
    "reference" : "Patient/example"
  },
  "effectiveDateTime" : "1999-07-02",
  "valueQuantity" : {
    "value" : 16.2,
    "unit" : "kg/m2",
    "system" : "http://unitsofmeasure.org",
    "code" : "kg/m2"
  },
  "derivedFrom" : [{
    "reference" : "Observation/bodyheight",
    "display" : "Body Height"
  },
  {
    "reference" : "Observation/example",
    "display" : "Body Weight"
  }]
}