diff --git a/lib/sympto/index.js b/lib/sympto/index.js
index d032965..b930638 100644
--- a/lib/sympto/index.js
+++ b/lib/sympto/index.js
@@ -16,9 +16,15 @@ export default function getSymptoThermalStatus({ cycle, previousCycle, earlierCy
   if (previousCycle) {
     const statusForLast = getSymptoThermalStatus({ cycle: previousCycle })
     if (statusForLast.temperatureShift) {
-      status.phases.preOvulatory = getPreOvulatoryPhase(cycle, [previousCycle, ...earlierCycles])
-      if (status.phases.preOvulatory.cycleDays.length === cycle.length) {
-        return status
+      const preOvuPhase = getPreOvulatoryPhase(
+        cycle,
+        [previousCycle, ...earlierCycles]
+      )
+      if (preOvuPhase) {
+        status.phases.preOvulatory = preOvuPhase
+        if (status.phases.preOvulatory.cycleDays.length === cycle.length) {
+          return status
+        }
       }
     }
   }
diff --git a/lib/sympto/pre-ovulatory.js b/lib/sympto/pre-ovulatory.js
index 01efb19..b1e7698 100644
--- a/lib/sympto/pre-ovulatory.js
+++ b/lib/sympto/pre-ovulatory.js
@@ -13,6 +13,9 @@ export default function(cycle, previousCycles) {
     return d.date <= preOvuEndDate
   })
   const preOvulatoryDays = getDaysUntilFertileMucus(maybePreOvuDays)
+  // if mucus occurs on the 1st cycle day, there is no pre-ovu phase
+  if (!preOvulatoryDays.length) return null
+
   let endDate
   if (preOvulatoryDays.length === maybePreOvuDays.length) {
     endDate = preOvuEndDate
diff --git a/test/sympto/fixtures.js b/test/sympto/fixtures.js
index 69792d5..6b61492 100644
--- a/test/sympto/fixtures.js
+++ b/test/sympto/fixtures.js
@@ -100,6 +100,29 @@ export const cycleWithEarlyMucus = [
   { date: '2018-06-27', temperature: 36.9, mucus: 4 }
 ].map(convertToSymptoFormat)
 
+export const cycleWithMucusOnFirstDay = [
+  { date: '2018-06-01', temperature: 36.6, bleeding: 2, mucus: 3},
+  { date: '2018-06-02', temperature: 36.65, mucus: 3 },
+  { date: '2018-06-05', temperature: 36.55 },
+  { date: '2018-06-06', temperature: 36.7, mucus: 0 },
+  { date: '2018-06-08', temperature: 36.45, mucus: 1 },
+  { date: '2018-06-09', temperature: 36.5, mucus: 4 },
+  { date: '2018-06-10', temperature: 36.4, mucus: 2 },
+  { date: '2018-06-11', temperature: 36.5, mucus: 3 },
+  { date: '2018-06-13', temperature: 36.45, mucus: 3 },
+  { date: '2018-06-14', temperature: 36.5, mucus: 4 },
+  { date: '2018-06-15', temperature: 36.55, mucus: 4 },
+  { date: '2018-06-16', temperature: 36.7, mucus: 3 },
+  { date: '2018-06-17', temperature: 36.65, mucus: 3 },
+  { date: '2018-06-18', temperature: 36.75, mucus: 4 },
+  { date: '2018-06-19', temperature: 36.8, mucus: 4 },
+  { date: '2018-06-20', temperature: 36.85, mucus: 4 },
+  { date: '2018-06-23', temperature: 36.9, mucus: 3 },
+  { date: '2018-06-24', temperature: 36.85, mucus: 4 },
+  { date: '2018-06-26', temperature: 36.8, mucus: 4 },
+  { date: '2018-06-27', temperature: 36.9, mucus: 4 }
+].map(convertToSymptoFormat)
+
 export const cycleWithoutAnyShifts = [
   { date: '2018-06-01', temperature: 36.6, bleeding: 2 },
   { date: '2018-06-02', temperature: 36.65 },
diff --git a/test/sympto/index.spec.js b/test/sympto/index.spec.js
index aed8f3a..7f65319 100644
--- a/test/sympto/index.spec.js
+++ b/test/sympto/index.spec.js
@@ -9,6 +9,7 @@ import {
   cycleWithoutAnyShifts,
   fiveDayCycle,
   cycleWithEarlyMucus,
+  cycleWithMucusOnFirstDay,
   mucusPeakAndFhmOnSameDay,
   fhmTwoDaysBeforeMucusPeak,
   fhm5DaysAfterMucusPeak,
@@ -453,6 +454,23 @@ describe('sympto', () => {
           })
       })
     })
+
+    it('shortens the pre-ovu phase if mucus occurs even on the first day', () => {
+      const status = getSensiplanStatus({
+        cycle: cycleWithMucusOnFirstDay,
+        previousCycle: fhmOnDay12,
+        earlierCycles: Array(10).fill(fhmOnDay12)
+      })
+
+
+      expect(Object.keys(status.phases).length).to.eql(1)
+
+      expect(status.phases.periOvulatory).to.eql({
+        start: { date: '2018-06-01' },
+        cycleDays: cycleWithMucusOnFirstDay
+      })
+    })
+
     it('lengthens the pre-ovu phase if >= 12 cycles with fhm > 13', () => {
       const status = getSensiplanStatus({
         cycle: longAndComplicatedCycle,