diff --git a/lib/sympto/index.js b/lib/sympto/index.js
index 3fd0102..b468f36 100644
--- a/lib/sympto/index.js
+++ b/lib/sympto/index.js
@@ -1,12 +1,19 @@
-import getTemperatureStatus from './temperature'
-import getMucusStatus from './mucus'
+import getTemperatureShift from './temperature'
+import getMucusShift from './mucus'
 
 export default function (cycleDays) {
-  const temperatureStatus = getTemperatureStatus(cycleDays)
-  const mucusStatus = getMucusStatus(cycleDays)
+  const assumeFertile = { assumeFertility: true }
+
+  const temperatureShift = getTemperatureShift(cycleDays)
+  if (!temperatureShift.detected) return assumeFertile
+
+  const tempEvalEndIndex = cycleDays.indexOf(temperatureShift.evaluationCompleteDay)
+  const mucusShift = getMucusShift(cycleDays, tempEvalEndIndex)
+  if (!mucusShift.detected) return assumeFertile
+
   return {
-    assumeFertility: true,
-    temperatureStatus,
-    mucusStatus
+    assumeFertility: false,
+    temperatureShift,
+    mucusShift
   }
 }
\ No newline at end of file
diff --git a/lib/sympto/mucus.js b/lib/sympto/mucus.js
index 0931ad6..ada8127 100644
--- a/lib/sympto/mucus.js
+++ b/lib/sympto/mucus.js
@@ -1,13 +1,25 @@
-export default function (cycleDays) {
+export default function (cycleDays, tempEvalEndIndex) {
   const mucusDays = cycleDays.filter(day => day.mucus && !day.mucus.exclude)
   const bestQuality = Math.max(...mucusDays.map(day => day.mucus.value))
-  const mucusPeak = mucusDays.find((day, i) => {
-    if (day.mucus.value !== bestQuality) return false
 
-    const threeFollowingDays = mucusDays.slice(i + 1, i + 4)
+  const mucusPeak = cycleDays.find((day, i) => {
+    if (!mucusDays.includes(day) || day.mucus.value !== bestQuality) return false
+
+    // sensiplan says the three following days must be of lower quality
+    // AND no best quality day may occur until temperature evaluation has
+    // been completed
+    const mucusDaysIndex = mucusDays.indexOf(day)
+    const threeFollowingDays = mucusDays.slice(mucusDaysIndex + 1, mucusDaysIndex + 4)
     if (threeFollowingDays.length < 3) return false
 
-    return threeFollowingDays.every(day => day.mucus.value < bestQuality)
+    const bestQualityOccurringIn3FollowingDays = threeFollowingDays.some(day => day.mucus.value >= bestQuality)
+    if (bestQualityOccurringIn3FollowingDays) return false
+
+    const relevantDays = cycleDays
+      .slice(i + 1, tempEvalEndIndex + 1)
+      .filter(day => day.mucus && !day.mucus.exclude)
+
+    return relevantDays.every(day => day.mucus.value < bestQuality)
   })
 
   if (!mucusPeak) return { detected: false }
@@ -17,3 +29,4 @@ export default function (cycleDays) {
     mucusPeak
   }
 }
+
diff --git a/lib/sympto/temperature.js b/lib/sympto/temperature.js
index 4779bc0..af90282 100644
--- a/lib/sympto/temperature.js
+++ b/lib/sympto/temperature.js
@@ -1,4 +1,4 @@
-export default function getTemperatureStatus(cycleDays) {
+export default function (cycleDays) {
   const temperatureDays = cycleDays
     .filter(day => day.temperature && !day.temperature.exclude)
     .map(day => {
diff --git a/test/sympto/index.spec.js b/test/sympto/index.spec.js
new file mode 100644
index 0000000..bbea1ae
--- /dev/null
+++ b/test/sympto/index.spec.js
@@ -0,0 +1,77 @@
+import chai from 'chai'
+import getSensiplanStatus from '../../lib/sympto'
+
+const expect = chai.expect
+
+function convertToSymptoFormat(val, i) {
+  return {
+    date: i,
+    temperature: val.temperature ? { value: val.temperature } : null,
+    mucus: val.mucus ? { value: val.mucus } : null
+  }
+}
+
+describe('sympto', () => {
+  describe('evaluating mucus and temperature shift together', () => {
+    it('reports fertile when mucus reaches best quality again within temperature evaluation phase', function () {
+      const values = [
+        { temperature: 36.6 },
+        { temperature: 36.65 },
+        { temperature: 36.5 },
+        { temperature: 36.6 },
+        { temperature: 36.55 },
+        { temperature: 36.7, mucus: 0 },
+        { temperature: 36.75, mucus: 0 },
+        { temperature: 36.45, mucus: 1 },
+        { temperature: 36.5, mucus: 4 },
+        { temperature: 36.4, mucus: 2 },
+        { temperature: 36.5, mucus: 3 },
+        { temperature: 36.55, mucus: 3 },
+        { temperature: 36.45, mucus: 3 },
+        { temperature: 36.5, mucus: 4 },
+        { temperature: 36.55, mucus: 4 },
+        { temperature: 36.7, mucus: 3 },
+        { temperature: 36.65, mucus: 3 },
+        { temperature: 36.75, mucus: 4 },
+        { temperature: 36.8, mucus: 1 },
+        { temperature: 36.85, mucus: 2 },
+        { temperature: 36.8, mucus: 2 },
+        { temperature: 36.9, mucus: 2 },
+        { temperature: 36.9, mucus: 1 },
+        { temperature: 36.85, mucus: 1 },
+        { temperature: 36.9, mucus: 1 },
+        { temperature: 36.8, mucus: 1 },
+        { temperature: 36.9, mucus: 1 }
+      ]
+
+      const temperatures = values.map(convertToSymptoFormat)
+      const status = getSensiplanStatus(temperatures)
+      expect(status).to.eql({
+        assumeFertility: false,
+        temperatureShift: {
+          detected: true,
+          ltl: 36.55,
+          rule: 0,
+          firstHighMeasurementDay: {
+            date: 15,
+            temperature: { value: 36.7 },
+            mucus: { value: 3 }
+          },
+          evaluationCompleteDay: {
+            date: 17,
+            temperature: { value: 36.75 },
+            mucus: { value: 4 }
+          }
+        },
+        mucusShift: {
+          detected: true,
+          mucusPeak: {
+            date: 17,
+            mucus: { value: 4 },
+            temperature: { value: 36.75 }
+          }
+        }
+      })
+    })
+  })
+})
\ No newline at end of file
diff --git a/test/sympto/mucus.spec.js b/test/sympto/mucus.spec.js
index c0fd2da..7e9b704 100644
--- a/test/sympto/mucus.spec.js
+++ b/test/sympto/mucus.spec.js
@@ -16,7 +16,7 @@ describe('sympto', () => {
       it('detects mucus shift correctly', function () {
         const values = [0, 0, 0, 1, 1, 2, 2, 2, 3, 3, 3, 2, 2, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0]
           .map(turnIntoCycleDayObject)
-        const status = getMucusStatus(values)
+        const status = getMucusStatus(values, 30)
         expect(status).to.eql({
           detected: true,
           mucusPeak: {
@@ -29,7 +29,7 @@ describe('sympto', () => {
       it('detects no mucus shift when there are less than 3 days of lower quality', function () {
         const values = [0, 1, 1, 2, 0, 0, 1, 2, 3, 2, 3, 3, 3, 2, 2]
           .map(turnIntoCycleDayObject)
-        const status = getMucusStatus(values)
+        const status = getMucusStatus(values, 30)
         expect(status).to.eql({ detected: false })
       })
 
@@ -41,7 +41,7 @@ describe('sympto', () => {
       it('detects no mucus shift when the mucus values are all the same', function () {
         const values = [2, 2, 2, 2, 2, 2, 2, 2]
           .map(turnIntoCycleDayObject)
-        const status = getMucusStatus(values)
+        const status = getMucusStatus(values, 30)
         expect(status).to.eql({ detected: false })
       })
     })