Intermittent fasting and fertility can interact in both directions. In people with overweight and insulin resistance — including many women with polycystic ovary syndrome (PCOS) — modest weight loss by any method, which may include fasting, can help ovulation return. In people who are lean, very active, or eating too little, fasting can add to an energy shortfall that disrupts menstrual cycles and ovulation. The starting point matters.
This page explains what the evidence suggests, the situations where fasting is likely to be a bad idea, what is known about male fertility, and general points for people trying to conceive. Research specifically on intermittent fasting and fertility is limited, so this is educational information only. If you are trying to conceive or have irregular cycles, talk to your doctor or fertility specialist before making changes.
Table of Contents
When Weight Loss May Support Fertility
There is good evidence that, in people with overweight or obesity, losing a modest amount of weight can improve ovulation and the chance of conception. Intermittent fasting is one possible way to achieve that; it has not been shown to be better than other approaches. Situations where this is relevant include:
- PCOS with overweight and insulin resistance. Insulin resistance contributes to the hormonal imbalance in PCOS. Losing around 5–10% of body weight is associated with more regular cycles in many women. Small studies of time-restricted eating in PCOS are encouraging but short-term.
- Overweight with irregular or absent ovulation. Modest weight loss restores ovulation in some women.
- Metabolic conditions. Type 2 diabetes, insulin resistance, and fatty liver are associated with reduced fertility; weight loss and better blood sugar control may help.
Whether fasting suits you is best decided with your doctor, especially as PCOS and fertility care may involve medication. See our PCOS guide for more.
When Fasting May Harm Fertility
Reproduction is sensitive to energy availability. Fasting is more likely to cause problems for:
- People who are already lean. Further calorie restriction can suppress ovulation and disrupt cycles.
- Anyone underweight (BMI under 18.5). Fasting is not advisable; eating enough is the priority.
- Athletes and people with high training loads. Those at risk of relative energy deficiency in sport (RED-S) should not add fasting.
- People with a history of hypothalamic amenorrhea. Even after recovery, this group may be vulnerable to recurrence with energy restriction.
- Demanding fasting schedules such as daily OMAD or frequent extended fasts, which make under-eating more likely.
- Anyone with a current or past eating disorder. Fasting is not recommended.
Hypothalamic Amenorrhea
Functional hypothalamic amenorrhea (HA) is the loss of periods because the brain reduces the hormonal signals that drive ovulation, usually in response to low energy availability, stress, or both. It is a common reason periods stop in people who under-eat or train heavily.
Contributing factors often include:
- Eating less energy than the body needs
- High exercise volume
- Low body weight or body fat
- Psychological stress
Restrictive fasting can contribute to several of these. Recovery usually involves eating more, often reducing exercise, and sometimes regaining weight, with medical support. Continuing to fast is likely to delay recovery.
If your periods have become irregular or stopped since you started fasting, stop fasting, eat regular adequate meals, and see your doctor — missed periods always deserve medical assessment, including ruling out pregnancy and other causes.
Male Fertility
Research on intermittent fasting and male fertility is limited. What is known:
- Studies of intermittent fasting and testosterone are small and mixed; some found no change and some found modest decreases, particularly in lean, active men
- Significant calorie restriction and rapid weight loss can lower testosterone
- There is little direct research on intermittent fasting and sperm quality
- In men with obesity, weight loss is associated with improvements in hormone levels and possibly sperm quality
For men trying to conceive, a moderate eating pattern that doesn’t cause large calorie deficits is the more cautious choice. If there are concerns about fertility, a medical assessment is the right first step.
If You’re Actively Trying to Conceive
General principles: talk to your doctor, change one thing at a time, and favour stability and good nutrition over rapid change. Most guidance also recommends starting a folic acid supplement before conception — ask your doctor or pharmacist about the right dose for you.
If you weren’t fasting before
It is generally better not to start while actively trying to conceive unless your doctor has suggested weight loss and agrees fasting is a reasonable way to do it.
If you were already fasting and your cycles are regular
Discuss it with your doctor. Many people choose to ease off to a gentler schedule. Avoid making it stricter.
If your cycles are irregular
Get a medical assessment first to find the cause. If you are lean or eating little, fasting is likely to be the wrong approach.
If you become pregnant
Stop fasting and return to regular meals. Pregnancy increases energy and nutrient needs, and intentional fasting is not recommended. Talk to your midwife or doctor about nutrition.
IVF and Assisted Reproduction
If you are having fertility treatment:
- Tell your fertility clinic about any fasting or dieting and follow their advice
- Many clinics advise against significant calorie restriction during treatment cycles
- Some clinics recommend weight loss before treatment for people with obesity; how to achieve it should be agreed with the clinic
- A balanced dietary pattern, such as a Mediterranean-style diet, is often suggested, although evidence on diet and IVF outcomes is still limited
Don’t start or continue fasting during active treatment without discussing it with your fertility specialist.
When to Stop Fasting
Stop fasting and talk to your doctor if:
- Your cycles become longer, irregular, or stop
- Ovulation tracking suggests reduced or absent ovulation
- You are losing weight you didn’t intend to lose, or are becoming underweight
- You are having a fertility evaluation or treatment
- You become pregnant
- You are breastfeeding (unless your healthcare provider advises otherwise)
Frequently Asked Questions
How long does it take for cycles to return after stopping fasting?
It varies widely. If under-eating suppressed your cycles, it can take several months of adequate eating, and sometimes longer. See your doctor if periods don’t return, as other causes need to be ruled out.
Can I do 16:8 while trying to conceive?
Ask your doctor. For some people with overweight and insulin resistance, a moderate schedule may be a reasonable way to manage weight. For people who are lean or have regular cycles, the cautious approach is to avoid strict fasting while trying to conceive.
Does fasting affect egg quality?
There is very little direct evidence. Obesity and insulin resistance are associated with poorer reproductive outcomes, and severe energy restriction may also be harmful, but whether intermittent fasting itself affects egg quality is unknown.
What about during pregnancy?
Intentional fasting is not recommended during pregnancy. Energy and nutrient needs increase, and a shortened eating window makes under-eating more likely. If you plan to observe a religious fast while pregnant, discuss it with your doctor or midwife first — many traditions allow exemptions during pregnancy.
After birth, when can I resume fasting?
Fasting is generally not recommended while breastfeeding, because milk production needs extra energy and fluid. If you are not breastfeeding, or have finished, talk to your doctor about when a gentle schedule is appropriate, and focus on recovery and nutrition first.
I’m a man wanting to support my fertility. Should I fast?
A moderate eating pattern for general health is reasonable for most men, but avoid large calorie deficits or long fasts while trying to conceive. Eat enough protein and a varied diet. If you have obesity, losing weight by a sustainable method may help, and your doctor can advise on the best approach.
The Bottom Line
Fasting’s relationship with fertility depends on the starting point. In people with overweight and insulin resistance, modest weight loss — which fasting can be one way to achieve — may support ovulation. In people who are lean, highly active, or under-eating, fasting can disrupt cycles. Evidence specific to intermittent fasting is limited. If you are trying to conceive, talk to your doctor before starting or continuing fasting, avoid strict schedules, and stop fasting during pregnancy and breastfeeding.