diff --git a/lib/cycle.js b/lib/cycle.js
index 872560d..dcac869 100644
--- a/lib/cycle.js
+++ b/lib/cycle.js
@@ -47,8 +47,8 @@ export default function config(opts) {
   function getPreviousCycle(dateString) {
     const cycleStart = getLastMensesStartForDay(dateString)
     if (!cycleStart) return null
-    const i = cycleDaysSortedByDate.indexOf(cycleStart)
-    const earlierCycleStart = cycleDaysSortedByDate[i - 1]
+    const i = cycleStartsSortedByDate.indexOf(cycleStart)
+    const earlierCycleStart = cycleStartsSortedByDate[i + 1]
     if (!earlierCycleStart) return null
     return getCycleForStartDay(earlierCycleStart)
   }
diff --git a/test/cycle.spec.js b/test/cycle.spec.js
index 6af3208..8508748 100644
--- a/test/cycle.spec.js
+++ b/test/cycle.spec.js
@@ -5,8 +5,6 @@ import cycleModule from '../lib/cycle'
 const expect = chai.expect
 chai.use(dirtyChai)
 
-// TODO getPreviousCycle
-
 describe('getCycleDayNumber', () => {
   it('works for a simple example', () => {
     const cycleStarts = [{
@@ -77,6 +75,179 @@ describe('getCycleDayNumber', () => {
 
 })
 
+describe('getPreviousCycle', () => {
+  it('gets previous cycle', () => {
+    const cycleDaysSortedByDate = [
+      {
+        date: '2018-07-05',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-06-05',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-05-05',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-05-04',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-05-03',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-04-05',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-04',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-03',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-02',
+        bleeding: { value: 2 }
+      },
+    ]
+
+    const cycleStarts = [
+      '2018-07-05',
+      '2018-06-05',
+      '2018-05-03',
+      '2018-04-02'
+    ]
+
+    const { getPreviousCycle } = cycleModule({
+      cycleDaysSortedByDate,
+      cycleStartsSortedByDate: cycleDaysSortedByDate.filter(d => {
+        return cycleStarts.includes(d.date)
+      })
+    })
+    const result = getPreviousCycle('2018-06-08')
+    expect(result).to.eql([
+      {
+        date: '2018-05-05',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-05-04',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-05-03',
+        bleeding: { value: 2 }
+      }
+    ])
+  })
+
+  it('returns null when target days is not in a cyle', () => {
+    const cycleDaysSortedByDate = [
+      {
+        date: '2018-07-05',
+      },
+      {
+        date: '2018-06-05',
+      },
+      {
+        date: '2018-05-05',
+      },
+      {
+        date: '2018-05-04',
+      },
+      {
+        date: '2018-05-03',
+      },
+      {
+        date: '2018-04-05',
+      },
+      {
+        date: '2018-04-04',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-03',
+      },
+      {
+        date: '2018-04-02',
+      },
+    ]
+
+    const cycleStarts = []
+
+    const { getPreviousCycle } = cycleModule({
+      cycleDaysSortedByDate,
+      cycleStartsSortedByDate: cycleDaysSortedByDate.filter(d => {
+        return cycleStarts.includes(d.date)
+      })
+    })
+    const result = getPreviousCycle('2018-06-08')
+    expect(result).to.eql(null)
+  })
+
+  it('returns null when there is no previous cycle', () => {
+    const cycleDaysSortedByDate = [
+      {
+        date: '2018-07-05',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-06-05',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-05-05',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-05-04',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-05-03',
+        bleeding: { value: 2 }
+      },
+      {
+        date: '2018-04-05',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-04',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-03',
+        mucus: { value: 2 }
+      },
+      {
+        date: '2018-04-02',
+        bleeding: { value: 2 }
+      },
+    ]
+
+    const cycleStarts = [
+      '2018-07-05',
+      '2018-06-05',
+      '2018-05-03',
+      '2018-04-02'
+    ]
+
+    const { getPreviousCycle } = cycleModule({
+      cycleDaysSortedByDate,
+      cycleStartsSortedByDate: cycleDaysSortedByDate.filter(d => {
+        return cycleStarts.includes(d.date)
+      })
+    })
+    const result = getPreviousCycle('2018-04-18')
+    expect(result).to.eql(null)
+  })
+})
+
 describe('getCyclesBefore', () => {
   it('gets previous cycles', () => {
     const cycleDaysSortedByDate = [