diff --git a/physics/discrete_trajectory_segment_body.hpp b/physics/discrete_trajectory_segment_body.hpp
index 0918b3d3d3..e690b3130e 100644
--- a/physics/discrete_trajectory_segment_body.hpp
+++ b/physics/discrete_trajectory_segment_body.hpp
@@ -353,11 +353,12 @@ void DiscreteTrajectorySegment<Frame>::ForgetBefore(Instant const& t) {
 template<typename Frame>
 void DiscreteTrajectorySegment<Frame>::ForgetBefore(
     typename Timeline::const_iterator const end) {
-  std::int64_t number_of_points_to_remove =
+  std::int64_t const number_of_points_to_remove =
       std::distance(timeline_.cbegin(), end);
-  number_of_dense_points_ =
-      std::max<std::int64_t>(
-          0, number_of_dense_points_ - number_of_points_to_remove);
+  std::int64_t const number_of_dense_points_to_remove = std::max<std::int64_t>(
+      0,
+      number_of_points_to_remove + number_of_dense_points_ - timeline_.size());
+  number_of_dense_points_ -= number_of_dense_points_to_remove;
 
   timeline_.erase(timeline_.cbegin(), end);
 }
@@ -542,9 +543,10 @@ void DiscreteTrajectorySegment<Frame>::WriteToMessage(
     downsampling_parameters_->tolerance.WriteToMessage(
         serialized_downsampling_parameters->mutable_tolerance());
   }
-  message->set_number_of_dense_points(
+  message->set_number_of_dense_points(std::min(
+      timeline_size,
       std::max<std::int64_t>(
-          0, number_of_dense_points_ - number_of_points_to_skip_at_end));
+          0, number_of_dense_points_ - number_of_points_to_skip_at_end)));
 
   // Convert the |exact| vector into a set, and add the extremities.  This
   // ensures that we don't have redundancies.  The set is sorted by time to
diff --git a/physics/discrete_trajectory_segment_test.cpp b/physics/discrete_trajectory_segment_test.cpp
index 3f13c25654..5657b59f6c 100644
--- a/physics/discrete_trajectory_segment_test.cpp
+++ b/physics/discrete_trajectory_segment_test.cpp
@@ -523,15 +523,17 @@ TEST_F(DiscreteTrajectorySegmentTest, SerializationRange) {
 
   serialization::DiscreteTrajectorySegment message1;
   circle1.WriteToMessage(&message1,
-                         /*begin=*/circle1.upper_bound(t0_ + 1 * Second),
-                         /*end=*/circle1.upper_bound(t0_ + 3 * Second),
+                         /*begin=*/circle1.upper_bound(t0_ + 4.9 * Second),
+                         /*end=*/circle1.upper_bound(t0_ + 4.98 * Second),
                          /*exact=*/{});
+  EXPECT_LE(message1.number_of_dense_points(), message1.zfp().timeline_size());
 
   serialization::DiscreteTrajectorySegment message2;
-  ForgetBefore(circle2.upper_bound(t0_ + 1 * Second)->time, circle2);
-  ForgetAfter(circle2.upper_bound(t0_ + 3 * Second)->time, circle2);
+  ForgetBefore(circle2.upper_bound(t0_ + 4.9 * Second)->time, circle2);
+  ForgetAfter(circle2.upper_bound(t0_ + 4.98 * Second)->time, circle2);
   circle2.WriteToMessage(&message2,
                          /*exact=*/{});
+  EXPECT_LE(message2.number_of_dense_points(), message2.zfp().timeline_size());
 
   // Writing a range of the segment is equivalent to forgetting and writing the
   // result.