diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 992002b81..c3966340a 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -595,23 +595,26 @@ class AssertionRewriter(ast.NodeVisitor):
         # docstrings and __future__ imports.
         aliases = [ast.alias(py.builtin.builtins.__name__, "@py_builtins"),
                    ast.alias("_pytest.assertion.rewrite", "@pytest_ar")]
-        expect_docstring = True
+        doc = getattr(mod, "docstring", None)
+        expect_docstring = doc is None
+        if doc is not None and self.is_rewrite_disabled(doc):
+            return
         pos = 0
-        lineno = 0
+        lineno = 1
         for item in mod.body:
             if (expect_docstring and isinstance(item, ast.Expr) and
                     isinstance(item.value, ast.Str)):
                 doc = item.value.s
-                if "PYTEST_DONT_REWRITE" in doc:
-                    # The module has disabled assertion rewriting.
+                if self.is_rewrite_disabled(doc):
                     return
-                lineno += len(doc) - 1
                 expect_docstring = False
             elif (not isinstance(item, ast.ImportFrom) or item.level > 0 or
                   item.module != "__future__"):
                 lineno = item.lineno
                 break
             pos += 1
+        else:
+            lineno = item.lineno
         imports = [ast.Import([alias], lineno=lineno, col_offset=0)
                    for alias in aliases]
         mod.body[pos:pos] = imports
@@ -637,6 +640,9 @@ class AssertionRewriter(ast.NodeVisitor):
                       not isinstance(field, ast.expr)):
                     nodes.append(field)
 
+    def is_rewrite_disabled(self, docstring):
+        return "PYTEST_DONT_REWRITE" in docstring
+
     def variable(self):
         """Get a new variable."""
         # Use a character invalid in python identifiers to avoid clashing.
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 2d61b7440..02270e157 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -65,13 +65,18 @@ class TestAssertionRewrite(object):
     def test_place_initial_imports(self):
         s = """'Doc string'\nother = stuff"""
         m = rewrite(s)
-        assert isinstance(m.body[0], ast.Expr)
-        assert isinstance(m.body[0].value, ast.Str)
-        for imp in m.body[1:3]:
+        # Module docstrings in 3.7 are part of Module node, it's not in the body
+        # so we remove it so the following body items have the same indexes on 
+        # all Python versions
+        if sys.version_info < (3, 7):
+            assert isinstance(m.body[0], ast.Expr)
+            assert isinstance(m.body[0].value, ast.Str)
+            del m.body[0]
+        for imp in m.body[0:2]:
             assert isinstance(imp, ast.Import)
             assert imp.lineno == 2
             assert imp.col_offset == 0
-        assert isinstance(m.body[3], ast.Assign)
+        assert isinstance(m.body[2], ast.Assign)
         s = """from __future__ import with_statement\nother_stuff"""
         m = rewrite(s)
         assert isinstance(m.body[0], ast.ImportFrom)
@@ -80,16 +85,29 @@ class TestAssertionRewrite(object):
             assert imp.lineno == 2
             assert imp.col_offset == 0
         assert isinstance(m.body[3], ast.Expr)
+        s = """'doc string'\nfrom __future__ import with_statement"""
+        m = rewrite(s)
+        if sys.version_info < (3, 7):
+            assert isinstance(m.body[0], ast.Expr)
+            assert isinstance(m.body[0].value, ast.Str)
+            del m.body[0]
+        assert isinstance(m.body[0], ast.ImportFrom)
+        for imp in m.body[1:3]:
+            assert isinstance(imp, ast.Import)
+            assert imp.lineno == 2
+            assert imp.col_offset == 0
         s = """'doc string'\nfrom __future__ import with_statement\nother"""
         m = rewrite(s)
-        assert isinstance(m.body[0], ast.Expr)
-        assert isinstance(m.body[0].value, ast.Str)
-        assert isinstance(m.body[1], ast.ImportFrom)
-        for imp in m.body[2:4]:
+        if sys.version_info < (3, 7):
+            assert isinstance(m.body[0], ast.Expr)
+            assert isinstance(m.body[0].value, ast.Str)
+            del m.body[0]
+        assert isinstance(m.body[0], ast.ImportFrom)
+        for imp in m.body[1:3]:
             assert isinstance(imp, ast.Import)
             assert imp.lineno == 3
             assert imp.col_offset == 0
-        assert isinstance(m.body[4], ast.Expr)
+        assert isinstance(m.body[3], ast.Expr)
         s = """from . import relative\nother_stuff"""
         m = rewrite(s)
         for imp in m.body[0:2]:
@@ -101,10 +119,14 @@ class TestAssertionRewrite(object):
     def test_dont_rewrite(self):
         s = """'PYTEST_DONT_REWRITE'\nassert 14"""
         m = rewrite(s)
-        assert len(m.body) == 2
-        assert isinstance(m.body[0].value, ast.Str)
-        assert isinstance(m.body[1], ast.Assert)
-        assert m.body[1].msg is None
+        if sys.version_info < (3, 7):
+            assert len(m.body) == 2
+            assert isinstance(m.body[0], ast.Expr)
+            assert isinstance(m.body[0].value, ast.Str)
+            del m.body[0]
+        else:
+            assert len(m.body) == 1
+        assert m.body[0].msg is None
 
     def test_name(self):
         def f():