Underweight Before Pregnancy: Why It Matters and How to Gain

Answer: being underweight before pregnancy (a BMI under 18.5) matters because it can upset periods and ovulation, is often linked with anaemia, and raises the chance of a small or early baby. The fix is a doctor’s check first, then a slow, food-first gain of about 0.25 to 0.5 kg a week, with folic acid started as your doctor advises before you try to conceive.

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Most pre-pregnancy advice in desi homes is about losing weight. Very little is said to the woman who is naturally slim, or who became thin through years of stress, a long illness, exam-season skipped meals or a strict diet before the wedding, and who now wants a baby but hears only “khao piyo, sab theek ho jayega” from relatives. Yet low weight is common in Pakistan and India, and it quietly affects periods, iron levels and the baby’s growth. This guide explains why, and gives a steady plan to reach a healthier weight before conceiving.

How to tell if you’re underweight: BMI in plain words

BMI (body mass index) compares weight to height. Simple. Work it out as weight in kilograms divided by height in metres, squared. For example, a woman of 1.58 m weighing 44 kg has a BMI of about 17.6, which is underweight. A healthy BMI is 18.5 to 24.9. Some South Asian guidelines use a slightly lower upper limit, but the underweight line stays at 18.5.

BMI isn’t perfect. Far from it. A woman at 18.3 who eats well, has regular periods and normal blood tests may simply be small-framed, like her mother and her nani before her, and nobody should push her to force down food she doesn’t need. Still, a BMI under 18.5 is worth a conversation with a doctor before pregnancy, and under 17 even more so.

Why low weight matters for fertility and pregnancy

Think of the body as a careful manager. When fat stores run low, it decides this isn’t a good time to grow a baby, much like a shopkeeper who won’t take a big new order when the storeroom is already half empty, and it quietly turns down the hormones that trigger ovulation each month. Periods become irregular, light or stop altogether. Our guide on underweight and missed periods explains the bone risk that comes with it too.

Low weight often travels with low iron. Often. A woman who starts pregnancy anaemic has less in reserve for the baby, who needs a lot of iron in the later months. And a mother who is underweight at the start, and doesn’t gain enough during pregnancy, has a higher chance of a baby born small or early. Our pregnancy weight gain chart by month shows that underweight women are actually advised to gain more during pregnancy than others, not less.

Did you know? A woman’s body usually needs a certain level of stored energy to keep ovulating regularly. That’s one reason female athletes and women with eating disorders often lose their periods.

See a doctor first: thinness can be a symptom

Before any gain plan, rule out a cause. Unexplained weight loss, or being unable to gain despite eating well, needs checking. Common culprits include an overactive thyroid, diabetes, TB, coeliac disease, worms and other gut problems, depression and eating disorders. Ask your doctor about:

• a haemoglobin or full blood count, for anaemia;
• thyroid tests (TSH);
• blood sugar;
• other tests your doctor thinks fit, based on your history.

Folic acid matters here too. A lot. Taking it from before conception protects the baby’s spine and brain as they form in the very first weeks, often before you’ve even noticed a missed period, let alone taken a test, which is exactly why doctors ask women who are planning a baby to start early rather than waiting for the good news. Your doctor will say which strength and when to start; some women need a higher strength than usual. Don’t wait for a positive test.

A gradual gain plan with desi food

Aim for 300 to 500 kcal a day above what you eat now. That usually gives about 0.25 to 0.5 kg a week, which builds real reserves rather than just water.

1. Eat every 3 hours: 3 meals and 2 or 3 snacks. Skipping breakfast is the most common gap.
2. Add, don’t replace: one extra roti or half a plate more rice at lunch and dinner, plus a teaspoon of ghee or oil in the salan.
3. Protein at each meal: 2 eggs, a katori of daal or chana, dahi, paneer, chicken or fish.
4. Two dense snacks: a banana milkshake (250 ml milk, 1 banana, a spoon of peanut butter or a few almonds), or 30 g of mixed nuts with 2 or 3 dates.
5. Iron foods daily: meat, liver, eggs, chana, rajma (always boiled well), palak, with a squeeze of lemon or some fruit alongside to help absorption.
6. Light strength work: 20 minutes, 3 times a week: sit-to-stand, wall push-ups, carrying water bottles. It helps weight land as muscle.
7. Weigh weekly: same scale, same morning. Adjust by a snack up or down every 2 weeks.

Keep chai for an hour or so after meals rather than with them, since the tannins in tea can reduce how much iron you absorb from the daal, chana and palak you’ve worked so hard to fit into the day, and that small change in timing costs nothing. More plans live in our Desi Weight Gain section.

Myth: gainer powders and fertility nuskhe speed things up

Unlabelled weight gainers and “wazan barhane wale” syrups sold online and by hakeems have repeatedly been found to contain hidden steroids or cyproheptadine. Neither belongs anywhere near a planned pregnancy. The same goes for herbal “fertility” mixes of unknown content. Food, time and a doctor’s checks are the safe route.

Did you know? You don’t need a “perfect” weight before trying. No. Many doctors are happy once periods are regular, blood counts are good and weight is heading up. Ask your doctor what target suits you.

When to start trying for a baby

Talk timing through with your doctor. Many women gain a few kilos, see periods settle into a regular pattern, fix anaemia, and are ready. If periods have been absent for 3 months or more, if you’ve been trying for a year without success (6 months if you’re over 35), or you have other concerns, ask for a fertility review. Our guide to causes of infertility in women explains what doctors check.

Questions women ask about being underweight before pregnancy