diff --git a/js/medicine/surgery/addGenitals/addOvaries.js b/js/medicine/surgery/addGenitals/addOvaries.js
index 65ed27678548d3151cd6cc6e6b4b1c18fc380c13..4432b54dd5e12b50ce7734415c70e176a124b258 100644
--- a/js/medicine/surgery/addGenitals/addOvaries.js
+++ b/js/medicine/surgery/addGenitals/addOvaries.js
@@ -97,6 +97,9 @@ App.Medicine.Surgery.Procedures.OFAddOvaries = class extends App.Medicine.Surger
 				this._slave.pubertyXX = 1;
 			}
 		}
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 
 		if (this.eggType === "human") {
 			return this._assemble(new App.Medicine.Surgery.Reactions.AddOvaries());
@@ -147,6 +150,9 @@ App.Medicine.Surgery.Procedures.OFReplaceOvaries = class extends App.Medicine.Su
 				this._slave.pubertyXX = 1;
 			}
 		}
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 
 		if (this.eggType === "human") {
 			return this._assemble(new App.Medicine.Surgery.Reactions.AddOvaries());
diff --git a/js/medicine/surgery/addGenitals/asexualReproOvaries.js b/js/medicine/surgery/addGenitals/asexualReproOvaries.js
index 737aa95b212f53817f2ec5a71121cce8048645bb..5b2b1135037263969d5b00f05396314048f1c3d3 100644
--- a/js/medicine/surgery/addGenitals/asexualReproOvaries.js
+++ b/js/medicine/surgery/addGenitals/asexualReproOvaries.js
@@ -57,6 +57,9 @@ App.Medicine.Surgery.Procedures.OFAsexualReproOvaries = class extends App.Medici
 	apply(cheat) {
 		if (this._slave.preg < -1) {
 			this._slave.preg = 0;
+			if (V.menstruation === 1) {
+				this._slave.fertPeak = 1;
+			}
 		}
 		this._slave.eggType = "human";
 		this._slave.pregData = _.cloneDeep(App.Data.misc.pregData.human);
diff --git a/js/medicine/surgery/addGenitals/freshOvaries.js b/js/medicine/surgery/addGenitals/freshOvaries.js
index ec557c0c7960e5f6051413af762ec6d09bc47809..8c9d34ef003814a9dc2d64515e2af495610dc5bb 100644
--- a/js/medicine/surgery/addGenitals/freshOvaries.js
+++ b/js/medicine/surgery/addGenitals/freshOvaries.js
@@ -50,6 +50,9 @@ App.Medicine.Surgery.Procedures.OFFreshOvaries = class extends App.Medicine.Surg
 				this._slave.pubertyXX = 1;
 			}
 		}
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.FreshOvaries());
 	}
 };
diff --git a/js/medicine/surgery/addGenitals/immortalOvaries.js b/js/medicine/surgery/addGenitals/immortalOvaries.js
index b62335ba319939af30bc168bacaa8bd4126284af..3fbeb98b7cfcffb1b34e3ee654d1546beeb15f55 100644
--- a/js/medicine/surgery/addGenitals/immortalOvaries.js
+++ b/js/medicine/surgery/addGenitals/immortalOvaries.js
@@ -57,6 +57,9 @@ App.Medicine.Surgery.Procedures.OFImmortalOvaries = class extends App.Medicine.S
 				this._slave.pubertyXX = 1;
 			}
 		}
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.ImmortalOvaries());
 	}
 };
diff --git a/js/medicine/surgery/addGenitals/mpreg.js b/js/medicine/surgery/addGenitals/mpreg.js
index a70a347e2594ef06d20704fbdda036719fa753a4..adcb211ce1e95e1dfff842e19e3eb3ba53042f28 100644
--- a/js/medicine/surgery/addGenitals/mpreg.js
+++ b/js/medicine/surgery/addGenitals/mpreg.js
@@ -66,6 +66,9 @@ App.Medicine.Surgery.Procedures.OFAddMPreg = class extends App.Medicine.Surgery.
 				this._slave.pubertyXX = 1;
 			}
 		}
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.AddMPreg());
 	}
 };
diff --git a/js/medicine/surgery/genitals/fertility.js b/js/medicine/surgery/genitals/fertility.js
index e421107cee4fd815bde80f5bd4b755643592632e..abb7ad5b1cbdd89184e10f1600aac617af40a947 100644
--- a/js/medicine/surgery/genitals/fertility.js
+++ b/js/medicine/surgery/genitals/fertility.js
@@ -38,6 +38,9 @@ App.Medicine.Surgery.Procedures.RestoreFertility = class extends App.Medicine.Su
 
 	apply(cheat) {
 		this._slave.preg = 0;
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.Fertility());
 	}
 };
diff --git a/js/medicine/surgery/genitals/mpregRemoved.js b/js/medicine/surgery/genitals/mpregRemoved.js
index 319dbf22562f114b3d037fb755240856538df8ce..d5416c61319452397eacd4e3c9ea8721494fd9ed 100644
--- a/js/medicine/surgery/genitals/mpregRemoved.js
+++ b/js/medicine/surgery/genitals/mpregRemoved.js
@@ -24,6 +24,9 @@ App.Medicine.Surgery.Procedures.RemoveMPreg = class extends App.Medicine.Surgery
 		this._slave.mpreg = 0;
 		this._slave.ovaImplant = OvaryImplantType.NONE;
 		this._slave.wombImplant = "none";
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.MPregRemoved());
 	}
 };
diff --git a/js/medicine/surgery/genitals/preg.js b/js/medicine/surgery/genitals/preg.js
index a1c108538d277a12247a8affe8a9699ed2f6718b..078a1132dd3498fefc81a972afb7e16cfaa83bda 100644
--- a/js/medicine/surgery/genitals/preg.js
+++ b/js/medicine/surgery/genitals/preg.js
@@ -77,6 +77,9 @@ App.Medicine.Surgery.Procedures.ImplantPregGenerator = class extends App.Medicin
 		this._slave.broodmother = 1;
 		this._slave.broodmotherFetuses = 1;
 		this._slave.pregControl = "none";
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 0;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.Preg());
 	}
 };
diff --git a/js/medicine/surgery/genitals/pregRemove.js b/js/medicine/surgery/genitals/pregRemove.js
index 8ed5c2e3f4579aab49ceddeb90ea69f8bdb7b8a8..ad56e6259e9f74ca30b37534c0a87ee2d56da2d7 100644
--- a/js/medicine/surgery/genitals/pregRemove.js
+++ b/js/medicine/surgery/genitals/pregRemove.js
@@ -47,6 +47,9 @@ App.Medicine.Surgery.Procedures.RemovePregGenerator = class extends App.Medicine
 		this._slave.broodmotherFetuses = 0;
 		this._slave.broodmotherOnHold = 0;
 		this._slave.pregControl = "none";
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.PregRemove());
 	}
 };
diff --git a/js/medicine/surgery/genitals/sterilize.js b/js/medicine/surgery/genitals/sterilize.js
index 8e822bd5c0951e39eae615acdd52a3798be827a6..52452311b8a51507a8cc673264353e404dadc5b4 100644
--- a/js/medicine/surgery/genitals/sterilize.js
+++ b/js/medicine/surgery/genitals/sterilize.js
@@ -41,6 +41,9 @@ App.Medicine.Surgery.Procedures.Sterilize = class extends App.Medicine.Surgery.P
 
 	apply(cheat) {
 		this._slave.preg = -2;
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.Sterilize());
 	}
 };
@@ -58,6 +61,9 @@ App.Medicine.Surgery.Procedures.Oophorectomy = class extends App.Medicine.Surger
 		this._slave.ovaries = 0;
 		this._slave.ovaImplant = OvaryImplantType.NONE;
 		this._slave.wombImplant = "none";
+		if (V.menstruation === 1) {
+			this._slave.fertPeak = 1;
+		}
 		return this._assemble(new App.Medicine.Surgery.Reactions.Sterilize());
 	}
 };
diff --git a/src/data/verification/verifyHumanState.js b/src/data/verification/verifyHumanState.js
index 31f28d85f6e64f477488a7c103fed834ca8f12a9..0ca9b779344bda98d3bda7c1900d9a78fd1b78f5 100644
--- a/src/data/verification/verifyHumanState.js
+++ b/src/data/verification/verifyHumanState.js
@@ -192,7 +192,7 @@ App.Verify.I.humanPregnancy = (actor, location) => {
 	if (actor.pubertyXX === 0 && (actor.ovaries > 0 || actor.mpreg > 0) && actor.preg === -1) {
 		actor.preg = 0; // no contraceptives for prepubescent humans
 	}
-	actor.fertPeak = Math.clamp(+actor.fertPeak, 0, 4) ?? 0;
+	actor.fertPeak = Math.clamp(+actor.fertPeak, -10, 10) ?? 0;
 	// @ts-expect-error Type 'number' is not assignable to type ...
 	actor.broodmother = Math.clamp(+actor.broodmother, 0, 3) ?? 0;
 	actor.broodmotherFetuses = Math.max(+actor.broodmotherFetuses, 0) ?? 0;
diff --git a/src/endWeek/nextWeek/nextWeek.js b/src/endWeek/nextWeek/nextWeek.js
index 133ab69c041cbfd0e4a3f04b4bde0d892f853690..a686a853d0afcf92ed35a469219c7aca2d870c9e 100644
--- a/src/endWeek/nextWeek/nextWeek.js
+++ b/src/endWeek/nextWeek/nextWeek.js
@@ -76,7 +76,28 @@ App.EndWeek.nextWeek = function() {
 		}
 	}
 	if (V.menstruation === 1) {
-		// TODO
+		if (V.PC.preg < -1 || (!V.PC.ovaries && !V.PC.mpreg) || V.PC.pubertyXX === 0 || V.PC.ovaryAge >= 47) {
+			// Handle sterility. Contraceptives will still continue the cycle.
+			V.PC.fertPeak = 1;
+		} else if (V.PC.geneticQuirks.superfetation === 2) {
+			if (V.PC.womb.length > 0) {
+				if (V.PC.fertPeak === 0) {
+					V.PC.fertPeak = 1;
+				}
+				V.PC.fertPeak--;
+			} else {
+				V.PC.fertPeak = 0;
+			}
+		} else if (V.PC.pregWeek < 0) {
+			V.PC.fertPeak = 2;
+		} else {
+			if (V.PC.fertPeak === 0) {
+				// Assume a standard 4 week cycle
+				V.PC.fertPeak += 3;
+			} else if (V.PC.fertPeak > 0) {
+				V.PC.fertPeak--;
+			}
+		}
 	} else if (V.PC.geneticQuirks.superfetation === 2 && V.PC.womb.length > 0) {
 		if (V.PC.fertPeak === 0) {
 			V.PC.fertPeak = 1;
@@ -200,7 +221,34 @@ App.EndWeek.nextWeek = function() {
 			}
 		}
 		if (V.menstruation === 1) {
-			// TODO
+			if (slave.preg < -1 || (!slave.ovaries && !slave.mpreg) || slave.pubertyXX === 0 || slave.ovaryAge >= 47) {
+				// Handle sterility. Contraceptives will still continue the cycle.
+				slave.fertPeak = 1;
+			} else if (slave.geneticQuirks.superfetation === 2) {
+				// Handle Superfetation
+				if (slave.womb.length > 0) {
+					if (slave.fertPeak === 0) {
+						slave.fertPeak = 1;
+					}
+					slave.fertPeak--;
+				} else {
+					slave.fertPeak = 0;
+				}
+			} else if (slave.preg > 0) {
+				slave.fertPeak = 2;
+			} else if (slave.pregWeek < 0 || slave.preg > 0) {
+				slave.fertPeak = 2;
+			} else {
+				if (slave.fertPeak === 0) {
+					// Assume a standard 4 week cycle
+					slave.fertPeak += 3;
+				} else if (slave.fertPeak > 0) {
+					slave.fertPeak--;
+				} else if (slave.fertPeak < 0) {
+					// This logics lets you go into the negatives. This allows multiple risky weeks in a row.
+					slave.fertPeak++;
+				}
+			}
 		} else if (slave.geneticQuirks.superfetation === 2 && slave.womb.length > 0) {
 			if (slave.fertPeak === 0) {
 				slave.fertPeak = 1;
diff --git a/src/facilities/surgery/analyzePregnancy.js b/src/facilities/surgery/analyzePregnancy.js
index 8f29becd3cb5f2d6dd39ec227c9bbdd38c650822..3c5198db72434e83467b315abd3000dd87ab814f 100644
--- a/src/facilities/surgery/analyzePregnancy.js
+++ b/src/facilities/surgery/analyzePregnancy.js
@@ -391,6 +391,8 @@ App.UI.analyzePregnancy = function() {
 		App.UI.DOM.appendNewElement("div", node, `Subject is in the postpartum period, and will regain fertility in ${num(slave.pregWeek * -1)} ${slave.pregWeek === -1 ? `week` : `weeks`}.`);
 	} else if (slave.preg === -1) { // special states
 		App.UI.DOM.appendNewElement("div", node, `Contraceptive agents detected in subject.`);
+	} else if (V.menstruation === 1 && slave.fertPeak > 0 && slave.preg === 0) {
+		App.UI.DOM.appendNewElement("div", node, `${He} is currently in ${his} safe week and not ovulating. ${His} next risky weak will be in ${num(slave.fertPeak)} week${slave.fertPeak > 1 ? 's' : ''}`);
 	}
 
 	return node;
diff --git a/src/facilities/surgery/surgeryPassageLower.js b/src/facilities/surgery/surgeryPassageLower.js
index 1ff5e3e4cb8f5a7cb8a3a08fcf56d9217e5bb477..4926fe32a8f23281e12916f8d335cdf466970ba7 100644
--- a/src/facilities/surgery/surgeryPassageLower.js
+++ b/src/facilities/surgery/surgeryPassageLower.js
@@ -387,7 +387,7 @@ App.UI.surgeryPassageLower = function(slave, refresh, cheat = false) {
 						App.UI.DOM.appendNewElement("div", el, `${His} indenture forbids elective surgery`, ["choices", "note"]);
 					} else if (slave.breedingMark === 1 && V.propOutcome === 1 && V.eugenicsFullControl !== 1 && FutureSocieties.isActive('FSRestart')) {
 						App.UI.DOM.appendNewElement("div", el, `${He} is protected from extreme surgery`, ["choices", "note"]);
-					} else if (isFertile(slave) && slave.ovaryAge <= 46) {
+					} else if (isFertile(slave, true) && slave.ovaryAge <= 46) {
 						r.push(`${He} could be made into a broodmother.`);
 						linkArray.push(App.Medicine.Surgery.makeLink(
 							new App.Medicine.Surgery.Procedures.ImplantPregGenerator(slave),
diff --git a/src/gui/options/options.js b/src/gui/options/options.js
index 2c7278ac5bfa7e5d390d8c27cbcf8676daa5b7f8..3b79aa9402f46278789be9c44aba1cfd8668e7aa 100644
--- a/src/gui/options/options.js
+++ b/src/gui/options/options.js
@@ -815,6 +815,10 @@ App.UI.optionsPassage = function() {
 			.addValue("Enabled", 1).on().addValue("Disabled", 0).off()
 			.addComment("This will sort rule assistant output. You may benefit if you have a lot of rules, but only want to look out for a specific portion of it.");
 
+		options.addOption("Slave Fertility Cycles", "menstruation")
+			.addValue("Enabled", 1).on().addValue("Disabled", 0).off()
+			.addComment("Adds slave fertility cycles. Slaves can only get pregnant on their fertile week. Fertility drugs can force slaves into a fertile week.");
+
 		options.addOption("Random slave events repeat control", "level", V.eventControl)
 			.addValue("No control", 0, () => V.eventControl.RIEPerWeek = Math.min(V.eventControl.RIEPerWeek, 3))
 			.addValue("Soft", 2, () => V.eventControl.RIEPerWeek = Math.min(V.eventControl.RIEPerWeek, 3))
diff --git a/src/interaction/siPhysicalRegimen.js b/src/interaction/siPhysicalRegimen.js
index 5b46d6c884cccad274c237eae6feeaaae777094d..1a6ab713ee352bb49c15f2b2f23ce9ad09158048 100644
--- a/src/interaction/siPhysicalRegimen.js
+++ b/src/interaction/siPhysicalRegimen.js
@@ -415,6 +415,8 @@ App.UI.SlaveInteract.physicalRegimen = function(slave, refresh) {
 					fertility = "using contraceptives";
 				} else if (slave.pregWeek < 0) {
 					fertility = "postpartum";
+				} else if (V.menstruation === 1 && slave.fertPeak > 0 && slave.preg === 0) {
+					fertility = "safe";
 				} else if (slave.preg === 0) {
 					fertility = "fertile";
 				} else if (slave.preg < 4 && (slave.broodmother === 0 || slave.broodmotherOnHold === 1)) {
diff --git a/src/interaction/siWork.js b/src/interaction/siWork.js
index 7065f9607994cfa08e6b482855a2a1dad9072821..09a58af3116ee20fde939ac7f5e2f9695ce54c88 100644
--- a/src/interaction/siWork.js
+++ b/src/interaction/siWork.js
@@ -416,7 +416,7 @@ App.UI.SlaveInteract.work = function(slave, refresh) {
 				sexOptions.push({text: `Get a footjob`, scene: () => App.Interact.fFeet(slave)});
 			}
 
-			if (canGetPregnant(slave) && (slave.geneticQuirks.superfetation !== 2 || V.geneticMappingUpgrade !== 0) && (slave.fuckdoll === 0) && V.seePreg !== 0) {
+			if (canGetPregnant(slave) && (slave.geneticQuirks.superfetation !== 2 || V.geneticMappingUpgrade !== 0 || slave.womb.length < 1) && (slave.fuckdoll === 0) && V.seePreg !== 0) {
 				if (canImpreg(slave, V.PC) || canFemImpreg(slave, V.PC)) {
 					sexOptions.push({
 						text: `Impregnate ${him} yourself`,
diff --git a/src/js/pregJS.js b/src/js/pregJS.js
index 6632fa4362e50561252553022e86b9f88ea32f18..51c7c62cb93cb9a0e2b9a44a75e8d6c4c434673a 100644
--- a/src/js/pregJS.js
+++ b/src/js/pregJS.js
@@ -408,22 +408,22 @@ globalThis.knockMeUp = function(target, chance, hole, fatherID) {
 				target.pregType = setPregType(target);
 				WombImpregnate(target, target.pregType, fatherID, 1);
 
-				if (V.menstruation === 1) {
-					//
+
+				target.pregKnown = 1;
+				if (target.ID === -1) {
+					/* r += "<span class="lime">You have gotten pregnant.</span>"; */
 				} else {
-					target.pregKnown = 1;
-					if (target.ID === -1) {
-						/* r += "<span class="lime">You have gotten pregnant.</span>"; */
+					const {He} = getPronouns(target);
+					r += `<span class="lime">${He} has become pregnant.</span>`;
+				}
+				if (V.menstruation === 1) {
+					target.fertPeak = 1;
+				}
+				if (target.geneticQuirks.superfetation === 2 && target.womb.length > 0) {
+					if (V.seeHyperPreg === 1) {
+						target.fertPeak = 1;
 					} else {
-						const {He} = getPronouns(target);
-						r += `<span class="lime">${He} has become pregnant.</span>`;
-					}
-					if (target.geneticQuirks.superfetation === 2 && target.womb.length > 0) {
-						if (V.seeHyperPreg === 1) {
-							target.fertPeak = 1;
-						} else {
-							target.fertPeak = 4;
-						}
+						target.fertPeak = 4;
 					}
 				}
 			}
@@ -488,26 +488,25 @@ globalThis.tryKnockMeUp = function(mother, chance, hole, father) {
 					mother.pregType = setPregType(mother);
 					WombImpregnate(mother, mother.pregType, father.ID, 1);
 
-					if (V.menstruation === 1) {
-						//
+					mother.pregKnown = 1;
+					if (mother.ID === -1) {
+						/* r += "<span class="lime">You have gotten pregnant.</span>"; */
 					} else {
-						mother.pregKnown = 1;
-						if (mother.ID === -1) {
-							/* r += "<span class="lime">You have gotten pregnant.</span>"; */
+						const {He, him} = getPronouns(mother);
+						if (father.ID === -1) {
+							r += `<span class="lime">${He} has become pregnant.</span>`;
 						} else {
-							const {He, him} = getPronouns(mother);
-							if (father.ID === -1) {
-								r += `<span class="lime">${He} has become pregnant.</span>`;
-							} else {
-								r += `<span class="lime">${father.slaveName} has gotten ${him} pregnant.</span>`;
-							}
+							r += `<span class="lime">${father.slaveName} has gotten ${him} pregnant.</span>`;
 						}
-						if (mother.geneticQuirks.superfetation === 2 && mother.womb.length > 0) {
-							if (V.seeHyperPreg === 1) {
-								mother.fertPeak = 1;
-							} else {
-								mother.fertPeak = 4;
-							}
+					}
+					if (V.menstruation === 1) {
+						mother.fertPeak = 1;
+					}
+					if (mother.geneticQuirks.superfetation === 2 && mother.womb.length > 0) {
+						if (V.seeHyperPreg === 1) {
+							mother.fertPeak = 1;
+						} else {
+							mother.fertPeak = 4;
 						}
 					}
 				}
@@ -580,6 +579,9 @@ globalThis.TerminatePregnancy = function(slave) {
 		// very early
 		slave.pregWeek = -1;
 	}
+	if (V.menstruation === 1) {
+		slave.fertPeak = 2;
+	}
 	WombFlush(slave);
 	SetBellySize(slave);
 };
diff --git a/src/js/slaveSummaryWidgets.js b/src/js/slaveSummaryWidgets.js
index e5a25768243ee1fb2f01208af8108b0898f839cc..98cef4072b64a17854777187a03775c38c570537 100644
--- a/src/js/slaveSummaryWidgets.js
+++ b/src/js/slaveSummaryWidgets.js
@@ -235,7 +235,9 @@ App.UI.SlaveSummaryRenderers = function() {
 				makeSpan(c, "Postpartum", styles);
 			} else if (slave.preg === -1) {
 				makeSpan(c, "CC", styles);
-			} else if (slave.preg === 0 && (slave.ovaries === 1 || slave.mpreg === 1)) {
+			} else if (V.menstruation === 1 && slave.fertPeak > 0 && slave.preg === 0 && (slave.ovaries === 1 || slave.mpreg === 1)) {
+				makeSpan(c, "Safe", styles);
+			}  else if (slave.preg === 0 && (slave.ovaries === 1 || slave.mpreg === 1)) {
 				makeSpan(c, "Fert+", styles);
 			} else if (((slave.preg < slave.pregData.normalBirth / 10) && (slave.preg > 0) && slave.pregKnown === 0) || slave.pregWeek === 1) {
 				makeSpan(c, "Preg?", styles);
@@ -581,6 +583,8 @@ App.UI.SlaveSummaryRenderers = function() {
 				}
 			} else if (slave.preg === -1) {
 				makeSpan(c, "On contraceptives.", styles);
+			}  else if (V.menstruation === 1 && slave.fertPeak > 0 && slave.preg === 0 && (slave.ovaries === 1 || slave.mpreg === 1)) {
+				makeSpan(c, "Safe.", styles);
 			} else if (slave.preg === 0 && (slave.ovaries === 1 || slave.mpreg === 1)) {
 				makeSpan(c, "Fertile.", styles);
 			} else if ((slave.preg >= 36) && (slave.broodmother > 0)) {
diff --git a/src/js/states/002-HumanState.js b/src/js/states/002-HumanState.js
index 55c3a203b0ecbbea5461e591e2aefeeb2da2cb20..24843c91ea59c226a70bf2444efd76df78f438d3 100644
--- a/src/js/states/002-HumanState.js
+++ b/src/js/states/002-HumanState.js
@@ -658,7 +658,7 @@ App.Entity.HumanState = class HumanState extends App.Entity.GenePoolRecord {
 		/**
 		 * Menstrual cycle control variable.
 		 *
-		 * * 0: Danger week
+		 * * 0-: Danger week
 		 * * 1+: safe week
 		 */
 		this.fertPeak = 0;
diff --git a/src/js/statsChecker/statsChecker.js b/src/js/statsChecker/statsChecker.js
index d4d72366a36afc3b9770178c0de7ef26e41951e6..f72785b623f094b33dfdb096aad20b5f6094165c 100644
--- a/src/js/statsChecker/statsChecker.js
+++ b/src/js/statsChecker/statsChecker.js
@@ -600,11 +600,13 @@ globalThis.canGetPregnant = function(slave) {
 	return false;
 };
 
-/** contraceptives (.preg === -1) do not negate this function
+/**
+ * contraceptives (.preg === -1) do not negate this function
  * @param {FC.HumanState} slave
+ * @param {boolean} ignoreCycle ignoreCycle=true will not take into account menstruation
  * @returns {boolean}
  */
-globalThis.isFertile = function(slave) {
+globalThis.isFertile = function(slave, ignoreCycle=false) {
 	if (!slave) {
 		return null;
 	}
@@ -631,10 +633,14 @@ globalThis.isFertile = function(slave) {
 		return false;
 	} else if (slave.mpreg === 1 || slave.ovaries === 1) {
 		if (slave.womb.length > 0) { // superfetation route
-			if (slave.fertPeak !== 0) {
+			if (slave.fertPeak > 0) {
 				return false;
 			}
 		}
+		// Menstruation route.
+		if (!ignoreCycle && V.menstruation === 1 && slave.fertPeak > 0 ) {
+			return false;
+		}
 		return true;
 	}
 	return false;
diff --git a/src/npc/descriptions/style/collar.js b/src/npc/descriptions/style/collar.js
index 953462bb634b467e1c8a8502bb5910c3a524956c..84a5c09801a510de7924e5241ab914f5af23f187 100644
--- a/src/npc/descriptions/style/collar.js
+++ b/src/npc/descriptions/style/collar.js
@@ -37,6 +37,8 @@ App.Desc.collar = function(slave) {
 						r.push(`"Womb not detected!"`);
 					} else if ((slave.ovaries > 0 || slave.mpreg > 0) && slave.pubertyXX === 0 && slave.physicalAge < V.fertilityAge) {
 						r.push(`"I'm not old enough to get pregnant yet!"`);
+					} else if (V.menstruation === 1 && slave.fertPeak > 0) {
+						r.push(`"I'm not ovulating right now!"`);
 					} else {
 						r.push(`"I'm infertile!"`);
 					}
@@ -73,6 +75,8 @@ App.Desc.collar = function(slave) {
 						}
 					} else if (isFertile(slave)) {
 						r.push(`"My womb needs filling!"`);
+					} else if (V.menstruation === 1 && slave.fertPeak > 0) {
+						r.push(`"My next risky period is in ${num(slave.fertPeak)} week${slave.fertPeak > 1 ? 's' : ''}!"`);
 					} else if ((slave.ovaries > 0 || slave.mpreg > 0) && slave.pubertyXX === 0) {
 						r.push(`"I should be fertile`);
 						if (slave.pubertyAgeXX - slave.physicalAge > 2) {
@@ -106,6 +110,8 @@ App.Desc.collar = function(slave) {
 						r.push(`"Put a baby in me today!"`);
 					} else if ((slave.ovaries > 0 || slave.mpreg > 0) && slave.pubertyXX === 0 && slave.physicalAge < V.fertilityAge) {
 						r.push(`"I'm too young to get pregnant!"`);
+					} else if (V.menstruation === 1 && slave.fertPeak > 0) {
+						r.push(`"This is a safe week!"`);
 					} else {
 						r.push(`"Try to knock me up some other time!"`);
 					}
diff --git a/src/npc/generate/slaveGenerationJS.js b/src/npc/generate/slaveGenerationJS.js
index 71ca61bbf845e116985bf39cc145c5b79a5d7bfe..99d1063db4ef96ee4c0331397e563b49184c0ea7 100644
--- a/src/npc/generate/slaveGenerationJS.js
+++ b/src/npc/generate/slaveGenerationJS.js
@@ -1443,6 +1443,9 @@ globalThis.generatePronouns = function(slave) {
 globalThis.generatePuberty = function(slave) {
 	if (((slave.ovaries === 1 || slave.mpreg === 1) && slave.physicalAge >= slave.pubertyAgeXX) || slave.pubertyXX === 1) {
 		slave.pubertyXX = 1;
+		if (V.menstruation === 1 && slave.geneticQuirks.superfetation < 2) {
+			slave.fertPeak = jsRandom(0, 3);
+		}
 	} else {
 		if (slave.preg >= -1) {
 			slave.preg = 0;
diff --git a/src/npc/surgery/surgery.js b/src/npc/surgery/surgery.js
index 5f6e3966780701f838a6d53688fb03782978f196..8ff0dde4acf3500b802967907a2d3d7a2f9ce0aa 100644
--- a/src/npc/surgery/surgery.js
+++ b/src/npc/surgery/surgery.js
@@ -561,7 +561,10 @@ globalThis.surgeryAmp = function(slave, part, cheat = false) {
 			}
 			if (slave.drugs === "clitoris enhancement" || slave.drugs === "intensive clitoris enhancement") {
 				slave.drugs = "no drugs";
-			}			
+			}
+			if (V.menstruation === 1) {
+				slave.fertPeak = 1;
+			}
 			surgeryDamage(slave, 20, cheat);
 			break;
 		case "voicebox":
diff --git a/src/player/electiveSurgery.js b/src/player/electiveSurgery.js
index 731891d40dbfef53e05866b4f189467c2229a84f..82366cfb28bd8b469a6fb50da57bb8b95affa0d3 100644
--- a/src/player/electiveSurgery.js
+++ b/src/player/electiveSurgery.js
@@ -3174,6 +3174,9 @@ App.UI.electiveSurgery = function() {
 			V.PC.preg = 0;
 			V.PC.ovaImplant = 0;
 			V.PC.wombImplant = "none";
+			if (V.menstruation === 1) {
+				V.PC.fertPeak = 1;
+			}
 			cashX(forceNeg(50000), "PCmedical");
 		})));
 		return el;
@@ -3190,6 +3193,9 @@ App.UI.electiveSurgery = function() {
 		el.append(App.UI.DOM.makeElement("div", surgeryLink("Fix your fertility concerns", "replaceOvaries", () => {
 			healthDamage(V.PC, 10);
 			V.PC.preg = 0;
+			if (V.menstruation === 1) {
+				V.PC.fertPeak = 1;
+			}
 			cashX(forceNeg(applyDiscount(500)), "PCmedical");
 		})));
 		return el;
@@ -3213,6 +3219,9 @@ App.UI.electiveSurgery = function() {
 				V.PC.eggType = "human";
 				V.PC.ovaImplant = 0;
 				V.PC.wombImplant = "none";
+				if (V.menstruation === 1) {
+					V.PC.fertPeak = 1;
+				}
 				cashX(forceNeg(50000), "PCmedical");
 			})));
 		}
@@ -3233,6 +3242,9 @@ App.UI.electiveSurgery = function() {
 			V.PC.ovaries = 1;
 			V.PC.preg = 0;
 			V.PC.eggType = "human";
+			if (V.menstruation === 1) {
+				V.PC.fertPeak = 1;
+			}
 			cashX(forceNeg(50000), "PCmedical");
 		}));
 		el.append(App.UI.DOM.makeElement("div", App.UI.DOM.generateLinksStrip(linkArray)));