I wanted to donate breast milk even before I had my now four-month-old daughter, Gallia, whose birth has made it possible. It is she, I suppose, who is performing the act of generosity, sharing her dinner with those not born so lucky. Gallia has recently acquired what I believe to be her fourth chin, and so she is in a position to be charitable. If that sounds smug, it isn’t. I am grateful beyond measure that I have been able to feed her. It wasn’t always thus.

Gallia’s older sisters, my identical twin girls, Raffaella and Celeste, now aged three, were born 10 weeks early. They needed oxygen to breathe, an incubator to regulate their temperature, intravenous nutritional support and a nasal-gastric tube that dripped breast milk directly into their unready stomachs to sustain them in place of their lost umbilical cord. They weighed just a shade over 2lb each and they would spend 56 days in hospital before they were well enough to come home.

I lay in the aftermath of an emergency caesarean while a gentle midwife showed me the distinctly ungentle manoeuvres required to hand-express. She squeezed and squeezed, but the tears that slid down my face came more readily than colostrum. My breasts had expected two more months of pregnancy; they were as unready as the rest of me. In the end the midwife left with 0.4ml in a tiny syringe. In the days that followed, the babies’ needs were so modest – 1ml an hour between them – but it was still more than my body would yield.

For two days my scant production was supplemented with donor milk. In their first days the feeding tube went into their mouths, for their nostrils were too little even for the tiniest of lines. A syringe was screwed on to the top of the tube, the plunger depressed, the whole “feed” over in moments – 0.5ml is about five drops, less than you would feed a day-old kitten. But those scant droplets were keeping my daughters alive. I stood by, feeling useless, guilty, grateful. I had failed to keep them safe inside my body and now I was failing to provide the scant nourishment they needed.

But somewhere out there I felt the presence of another mother – and I felt supported, nurtured by her care. Here, take this. I have, you need. I felt so close to her I could almost see her face. She was anonymous, yet there were things I knew about her: she had a new baby. Without doubt, she had her own worries. Yet her generosity made possible the miracles that were my daughters. She offered up her antibodies to newborns in a state of terrifying immune compromise. She held my hand and we shielded the babies as best we could from necrotising enterocolitis (death of the bowel) and from sepsis, from deadly bacteria and viruses. Because of her, I was granted space to heal, to catch up, to learn.

I expressed and expressed. I Googled and cried, and then I expressed again. There was a surprisingly robust uniformity of opinion on the internet, and sources ranging from science to quackery. But it was generally agreed that the following are good for the production of breast milk: malt, porridge oats, dark beer, garlic, black sesame seeds, carrot juice, barley, asparagus, brown rice, almonds, alfalfa, fenugreek. As two-thirds of my meals came from a hospital vending machine I opted for supplements, ignoring “directions for use” and swallowing fenugreek capsules until I began to smell like a chicken curry. Spice permeated my skin; my breath and sweat took on its tang. Who knows if it worked, but my milk soon came in – probably it always would have done.

After a day or two we no longer needed donor milk. But those days were not an inconsiderable gift: research has shown that for the tiniest premature babies, born with immature guts and immune systems, formula milk increases the incidence of necrotising enterocolitis (NEC), one of the most feared killers in neonatal intensive care. In 2016, a study in the Journal of Pediatrics found that formula-fed infants were at a higher risk of developing NEC compared with those who were breastfed, and an article the same year in Pediatrics found specifically that the increased availability of donor milk is associated with reduced incidence of NEC.

Donating breast milk isn’t widely publicised and is woefully under-funded. It is a constant frustration to services that there are more than enough willing milk donors around the country, but the banks do not have the financial wherewithal to process, store and distribute sufficient stock. Despite the well-established proven health benefits of breast milk for both pre-term and full-term infants, there is not the funding to ensure equal distribution around the country. Access is patchwork, dependent upon local trusts and policies.

Donating breast milk is not the same as milk sharing, a practice as ancient as wet nursing. The Facebook page Human Milk 4 Human Babies UK has 23k likes, and steady traffic around the country offering and requesting milk. But while milk banks screen and pasteurise, milk sharing is informal and, crucially, unscreened for viruses and pathogens. Exclusive breastfeeding is what we are told, endlessly, is best, and an inevitable consequence of such educational campaigns is that mothers who struggle to nurse their babies are left feeling inadequate, without necessarily having access to the support that might enable them to overcome their early difficulties.

But new models might permit safe provision to home and hospital. The Hearts Milk Bank provides milk to neonatal units and to mothers struggling to feed. Priority goes to women who cannot feed due to chemotherapy, mastectomy or illness; and to babies with bowel or terminal illnesses who cannot tolerate other food. With greater funding, they hope to be able to provide lactation support and safe donor milk to anyone who needs it. “Ninety per cent of mothers who want to breastfeed stop before they intended to,” says Natalie Shenker, co-founder of Hearts. “If everyone had support with breastfeeding after giving birth, including access to a lactation consultant and donor milk where necessary, breastfeeding rates in the UK could be transformed.”

Thinking I was an old hand at the pumping game, I didn’t register as a donor until I had the requisite two litres in the freezer. I was familiar with the hygiene protocols required from my own days expressing in a hospital, so I thought I knew what I was doing. I filled out a lifestyle questionnaire and had a chat with a nurse on the phone. It all looks good, she told me. They would send a courier for the milk and once they had received it I would go to my local hospital for a blood test: HIV, syphilis, hepatitis B and C. Just one more formality – what temperature was my freezer? It has to be -18C, she said, or colder. Still on the phone, I padded into the kitchen where -16C was illuminated in red. I peered sorrowfully into the bottom drawer in which 37 bags of milk were stacked between boxes of fish fingers and Soleros. “Yes, of course.” Softly I closed the door, and took a breath. “Minus 18. No problem at all. I’ll start again.” Yes, but just “think of the calories,” said a friend.

I write this, as I write much else, attached to a breast pump. They have come on light years: these days I have no ghastly ca-thunking locomotive soundtrack, no Carry On protuberances, no flanges. Breast pumps have always been ludicrous: one part agricultural, two parts penis extender. In the milking shed of the newborn intensive care unit, we sat in a circle, tits out, conversation punctuated by the obscene slurp and suck, and the wheeze of the motors. Occasionally someone would turn theirs up too high and yelp; other times we would get distracted, chatting, and someone else would begin to overflow, and make the inevitable spilled milk joke, even as the tears began to fall.

Now that is behind me, as is the conical Madonna affair that used to so traumatise the Amazon man when I answered the door. Today I have a neat little Elvie pump tucked into my bra, understated and ergonomic as a sex toy; wireless, USB-chargeable, Bluetooth enabled. If you didn’t know it was there, you might think I was just busty.

My twins are bonny and well. The trauma of their first months is behind us, but I will never forget it, nor would I want to. I pump after Gallia has had her morning feed, and then in the evenings once she’s down and I have collapsed on the sofa; I wander around the house on the classic parents’ nightly perambulation, picking up Duplo, recovering abandoned socks. It has so easily become part of my routine that I barely think about it; five minutes here, 10 minutes there, not enormous amounts, but a slow accretion in the freezer, until sooner than you’d think it is time to call the courier. I am fulfilling a promise I made to myself, long ago.

Mother Ship by Francesca Segal is published by Chatto & Windus at £14.99; or £11.49 from guardianbookshop.com. Visit Hearts Milk Bank at heartsmilkbank.org