diff --git a/index.html b/index.html
index 35caffa..19c7a6e 100644
--- a/index.html
+++ b/index.html
@@ -266,7 +266,7 @@ button{font-family:inherit;}
   text-transform:uppercase;letter-spacing:0.05em;margin-bottom:8px;font-weight:600;}
 input[type="range"].week-slider{
   width:100%;-webkit-appearance:none;appearance:none;height:8px;border-radius:999px;
-  background:linear-gradient(to right, var(--pregnancy-soft) 0%, var(--pregnancy-soft) 100%);
+  background:linear-gradient(to right, var(--pregnancy) 0, var(--pregnancy) calc(11px + (100% - 22px) * var(--f,0)), var(--pregnancy-soft) calc(11px + (100% - 22px) * var(--f,0)), var(--pregnancy-soft) 100%);
   outline:none;cursor:pointer;
 }
 input[type="range"].week-slider::-webkit-slider-thumb{
@@ -1074,3 +1074,4 @@ footer.note{
 <script src="vendor/three.min.js"></script>
 <script src="ring3d.js"></script>
 <script src="animations.js"></script>
+<script src="timeline.js"></script>
diff --git a/timeline.js b/timeline.js
new file mode 100644
index 0000000..59cb6d0
--- /dev/null
+++ b/timeline.js
@@ -0,0 +1,68 @@
+/* CycleWise AU - pregnancy timeline motion (Step 3).
+   - The week slider's track fills with colour up to the current week.
+     It grows the first time you scroll to it, then follows every week change.
+   - The baby-size picture pops a little when the week changes.
+   - The 40 week buttons wave in the first time they appear.
+   Cosmetic only: without GSAP the slider simply shows its normal look. */
+(function(){
+  "use strict";
+  var slider = document.getElementById("week-slider");
+  var badge = document.getElementById("wk-badge");
+  if (!slider || !badge) return;
+
+  var hasGsap = !!window.gsap;
+  var reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
+  var animate = hasGsap && !reduce;
+
+  var fill = { f: 0 };
+  var target = 0;
+  var introDone = false;
+
+  function paint(){ slider.style.setProperty("--f", fill.f.toFixed(4)); }
+  function currentWeek(){
+    var n = parseInt((badge.textContent || "").replace(/\D/g, ""), 10);
+    return isNaN(n) ? (parseInt(slider.value, 10) || 1) : Math.max(1, Math.min(40, n));
+  }
+  function goTo(f){
+    target = f;
+    if (!introDone) return;                 // wait until the slider is on screen
+    if (!animate) { fill.f = f; paint(); return; }
+    gsap.to(fill, { f: f, duration: 0.6, ease: "power2.out", overwrite: true, onUpdate: paint });
+  }
+
+  // Follow every week change, whether from the slider, a week button or the calendar.
+  var lastWeek = currentWeek();
+  target = (lastWeek - 1) / 39;
+  new MutationObserver(function(){
+    var w = currentWeek();
+    if (w === lastWeek) return;
+    lastWeek = w;
+    goTo((w - 1) / 39);
+    var blob = document.querySelector("#wk-blob svg");
+    if (animate && blob) {
+      gsap.fromTo(blob, { scale: 0.8, transformOrigin: "50% 50%" },
+        { scale: 1, duration: 0.5, ease: "back.out(2)", clearProps: "transform" });
+    }
+  }).observe(badge, { childList: true, characterData: true, subtree: true });
+
+  // First time the slider is visible: grow the fill from empty up to this week.
+  new IntersectionObserver(function(entries, obs){
+    if (!entries[0].isIntersecting) return;
+    obs.disconnect();
+    introDone = true;
+    if (!animate) { fill.f = target; paint(); return; }
+    gsap.to(fill, { f: target, duration: 1.2, ease: "power2.out", onUpdate: paint });
+  }).observe(slider);
+
+  // First time the week buttons are visible: wave them in.
+  var strip = document.getElementById("week-filmstrip");
+  if (strip && animate) {
+    new IntersectionObserver(function(entries, obs){
+      if (!entries[0].isIntersecting) return;
+      obs.disconnect();
+      gsap.from(strip.querySelectorAll(".film-item"), {
+        opacity: 0, scale: 0.8, duration: 0.4, stagger: 0.02, ease: "power2.out", clearProps: "all"
+      });
+    }).observe(strip);
+  }
+})();