Why menstruation and haemoglobin are connected

Menstruation involves monthly blood loss, and with it a loss of iron. Iron is the mineral the body uses to produce haemoglobin, the protein in red blood cells that carries oxygen. When iron stores are depleted, haemoglobin production falls. For donors with light to moderate periods, the monthly iron loss is typically replenished through diet between cycles and has little practical effect on donation eligibility.

For donors with heavy or prolonged periods, also known as menorrhagia, the monthly blood loss is significantly greater and the body may not fully replenish iron stores before the next cycle begins. In these cases, haemoglobin levels may be genuinely and consistently lower, making deferral on the day more likely regardless of when in the cycle the appointment falls.

The haemoglobin threshold and what it means

Before every donation, staff check haemoglobin concentration using a finger-prick blood test. The minimum threshold for female donors is 125 grams per litre. For male donors the threshold is 135 g/L. The difference reflects normal physiological variation between sexes.

If your haemoglobin reading is below 125 g/L on the day, you will be deferred and asked to return when levels have recovered. This is a temporary deferral, not a permanent one. You are given information about iron and recovery and can rebook at a later date. Repeated deferrals for low haemoglobin are a signal worth discussing with your GP.

Best timing within your cycle

If your periods are heavy, the one to two weeks after your period ends is typically when iron stores are at their highest point in the cycle and haemoglobin levels are most likely to be above the threshold. Mid-cycle is generally the optimal window for donation if haemoglobin has been an issue in the past.

If your periods are light and you have never been deferred for low haemoglobin, the timing of your appointment within your cycle matters much less. Many donors with light periods donate during their period routinely without any difficulty.

Iron-rich foods and how to use them

Eating iron-rich foods in the days before your appointment is one of the most effective practical steps you can take to support a successful haemoglobin reading. Good dietary sources of haem iron, which is the most easily absorbed form, include red meat, liver and other organ meats, and oily fish. Non-haem iron from plant sources is less easily absorbed but still valuable: dark leafy vegetables, lentils, chickpeas, tofu, pumpkin seeds, quinoa and fortified cereals all contribute.

Vitamin C significantly enhances the absorption of non-haem iron. Drinking a glass of orange juice with an iron-rich meal, or eating fruit alongside plant-based iron sources, improves how much iron your body extracts. Conversely, tea, coffee and calcium-rich foods consumed at the same meal reduce iron absorption. Save these for between meals rather than alongside your pre-donation iron-boosting foods.

The donation interval and why women wait longer

Women can give blood every 16 weeks. Men can give blood every 12 weeks. This longer interval for women is not arbitrary. It reflects the additional iron demands placed on the body by menstruation. The extra four weeks gives the body more time to replenish iron stores between donations.

Women who donate regularly at the maximum 16-week frequency should monitor their iron status. The combination of monthly blood loss and regular donation can cumulatively deplete iron reserves over time, particularly for those with heavier periods or restricted diets. Symptoms of low iron include persistent fatigue, breathlessness on mild exertion, pale skin and cold hands and feet. If these appear, speak to your GP.