For mothers feeding a baby or toddler at the breast and discovering they are pregnant again, the body’s workload changes overnight. Most do not notice it until the tiredness arrives, or the hunger, or the unfamiliar nausea during a feed. Understanding what is physiologically going on can take some of the confusion out of the next nine months.
What it is and why it matters
Breastfeeding while pregnant is a state where two major physiological processes are running at once. Lactation continues to be supported by elevated prolactin, which maintains milk production and the transfer of nutrients into breastmilk. At the same time, pregnancy is being driven by progesterone, oestrogen, and human chorionic gonadotrophin (hCG), each supporting placental development, the uterine environment, and the maternal adaptations needed to grow a new baby (Marshall et al., 2022). The body is not choosing between the two processes. It is doing both.
What the research shows
Energy demand increases across both processes. Pregnancy alone raises maternal energy requirements by roughly 300 kcal per day across the second and third trimesters (Kominiarek and Rajan, 2016). Lactation adds approximately another 500 kcal per day, accounting for the energy cost of milk production minus the small contribution from any remaining fat stores (WHO/FAO, 2004). When both processes are active, this can mean an additional 700 to 900 kcal per day, often more depending on how often the older child is feeding and whether they are still relying on breastmilk for their primary nutrition.
This baseline does not capture the broader load. Disrupted sleep, ongoing physical recovery from the previous birth, and the demands of caring for young children all sit on top of the metabolic equation.
Nutrient demand rises across both processes. Both pregnancy and lactation pull from the same nutrient stores. Research highlights several nutrients of particular concern during this phase, including protein for tissue and milk production, calcium for skeletal support and milk content, iron for blood volume expansion, iodine for thyroid function and fetal development, and zinc and magnesium for cellular growth, repair, and nervous system regulation (Marangoni et al., 2016; Marshall et al., 2022). Glucose demand also rises, supplying fetal development, milk production, and maternal energy at the same time. When intake does not match demand, the body draws from maternal reserves first, which is why breastfeeding while pregnant fatigue often feels deeper than ordinary tiredness.
The nervous system is also adapting. Sleep is fragmented by night feeds. The metabolic system is running at a higher idle. Caregiving is constant. Research consistently identifies physiological depletion during this phase as a contributing factor to fatigue, lower mood, and reduced stress tolerance (Marshall et al., 2022).
Strategies to support the body through this phase
- Eat more, more often. The Australian Breastfeeding Association recommends eating a little more and choosing healthy foods during this phase. Smaller meals and snacks every two to three hours often work better than three large meals. Pair protein, complex carbohydrate, and a healthy fat at each one.
- Prioritise mineral-rich foods. Slow-cooked meats, eggs, dairy if tolerated, sardines, leafy greens, legumes, oats, and nuts cover most of the nutrients drawn most heavily during this phase. Pair iron-rich foods with vitamin C to improve absorption.
- Drink to thirst. Lactation increases water needs. Keeping a water bottle within reach during feeds prevents the dehydration headaches common in the first trimester.
- Continue a quality prenatal supplement. Most lactation researchers recommend continuing a prenatal vitamin throughout breastfeeding, particularly when pregnant again, to cover iodine, folate, and B-complex needs (Kominiarek and Rajan, 2016).
- Reduce overall load where possible. Saying no to non-essential commitments, accepting offered help, and lowering the bar on housework all create space for rest.
- Rest where the day allows. Lying down to feed, going to bed earlier, and using daytime quiet hours rather than scrolling them can recover meaningful energy across a week.
What this can look like
A mum feeding her 14-month-old around four times a day discovers she is six weeks pregnant. The first trimester nausea makes eating harder, and her toddler is feeding more than usual. Rather than try to hold a perfect three-meal pattern, she keeps a tin of nuts and a jar of dates within reach, has a boiled egg with toast for breakfast, includes meat or fish at lunch, and accepts a friend’s offer to drop over a slow-cooked meal on Tuesdays. By the start of the second trimester, the deeper fatigue lifts, and she stops feeling like she is running on empty.
When to seek support
Persistent vaginal bleeding, unusual cramping during a feed, a history of miscarriage or preterm labour, or a noticeable drop in maternal weight all warrant a conversation with a GP, midwife, or maternal child health nurse. Most pregnancies tolerate continued breastfeeding well, but personalised guidance matters when these factors are present (Australian Breastfeeding Association, n.d.).
A closing thought
Breastfeeding while pregnant is not something to optimise. It is a phase where the body is being asked to do more, and the most useful response is to recognise the demand and meet it gently, with adequate food, mineral-rich meals, rest where possible, and a quieter overall load.
References
Australian Breastfeeding Association. (n.d.). Breastfeeding through pregnancy. https://www.breastfeeding.asn.au/resources/breastfeeding-through-pregnancy
Kominiarek, M.A., and Rajan, P. (2016). Nutrition recommendations in pregnancy and lactation. Medical Clinics of North America, 100(6), 1199 to 1215.
Marangoni, F., Cetin, I., Verduci, E., Canzone, G., Giovannini, M., Scollo, P., Corsello, G., and Poli, A. (2016). Maternal diet and nutrient requirements in pregnancy and breastfeeding: An Italian Consensus Document. Nutrients, 8(10), 629.
Marshall, N.E., Abrams, B., Barbour, L.A., Catalano, P., Christian, P., Friedman, J.E., Hay, W.W., Hernandez, T.L., Krebs, N.F., Oken, E., Purnell, J.Q., Roberts, J.M., Soltani, H., Wallace, J., and Thornburg, K.L. (2022). The importance of nutrition in pregnancy and lactation: lifelong consequences. American Journal of Obstetrics and Gynecology, 226(5), 607 to 632.
World Health Organization and Food and Agriculture Organization (WHO/FAO). (2004). Human energy requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. FAO Food and Nutrition Technical Report Series 1.