Pregnancy Week 25: Baby's Lung Development

You're 25 weeks pregnant — deep into the second trimester, closer to meeting your baby than not, and firmly in the "look at that bump" territory. Kicks are strong, your body is unmistakably in full pregnancy mode, and inside, your baby is doing some of the most important quiet work of the whole 40 weeks: building their lungs.

This week's guide walks you through what your baby is up to, what your body is doing, and the sensible small things — nutrition, movement, questions for your next appointment, and a handful of practical tips. There's no big milestone requiring action; the assignment is really just to keep going, be kind to yourself and take an occasional breath.

Standard note first: this article is educational, not medical advice. Every pregnancy is different, and your obstetrician or midwife knows yours. We're the team behind Vyve, a private AI pregnancy tracker — so, transparent source — but nothing here replaces your clinician.

Your baby this week

At 25 weeks, your baby is roughly the size of a cauliflower (or a rutabaga, depending on your produce section) — about 34 centimeters (around 13.5 inches) from head to heel and weighing approximately 660 to 700 grams, about 1.5 pounds. They're lengthening steadily, and week by week, they're starting to plump up as more fat builds under the skin.

What else is happening this week:

• Skin. Skin is still wrinkled and translucent, but it's smoothing gradually as fat accumulates underneath. It's also starting to develop pigmentation.
• Hair. Actual hair color and texture are becoming apparent, though this is only truly visible on ultrasound and even then not always. Fine lanugo still covers most of the body.
• Nostrils. Your baby's nostrils, which had been closed until recently, are now open. Combined with rhythmic "practice breathing" of amniotic fluid, this helps their respiratory system get ready.
• Grasp reflex. Tiny hands are curling into fists and grasping — often the umbilical cord or their own feet. Startlingly cute if you catch it on a scan.
• Sense of movement and balance. With the inner ear's balance system now mature, your baby is starting to sense which way is up.
• Response to sound. Your baby recognizes your voice and often responds to familiar sounds — some parents notice a favorite song or a partner's voice quiets or excites them.

Lung development — why this week matters

If there's a headline milestone for week 25, it's the lungs. Your baby's respiratory system has been developing for months, but this is a genuinely important stretch.

Here's what's under way:

• Alveoli. The tiny air sacs at the ends of the airways — where oxygen will eventually cross into the blood after birth — are multiplying rapidly. This alveolar branching continues right up to and after birth.
• Surfactant. Specialized cells (called type II pneumocytes) begin producing surfactant, a substance that reduces surface tension inside the tiny air sacs and stops them from collapsing when your baby exhales after birth. Adequate surfactant is one of the biggest reasons prematurity outcomes have improved so much — modern NICUs can supplement it when it's still developing.
• Capillaries. A dense network of tiny blood vessels is weaving in around the developing air sacs, ready to shuttle oxygen from lungs to blood.
• Practice breathing. Your baby is inhaling and exhaling amniotic fluid, which helps the lungs mature and strengthens the muscles that will drive breathing.

Lungs aren't fully ready yet — that's a process that continues into the third trimester and even after birth. But the architecture is now clearly recognizable, and each additional week from here makes a real difference in how ready they'll be for that first breath.

The most important work at 25 weeks happens quietly — tiny air sacs multiplying, surfactant starting up, capillaries weaving in. You can't feel it. It's still one of the biggest weeks of the whole pregnancy.

Mom's body & symptoms

Your uterus now reaches roughly halfway between your belly button and your ribcage, and it's still growing week by week. Total weight gain is often in the 15–22 pound range at this stage, though the healthy range varies quite a bit by individual.

Common week-25 symptoms:

• Strong, purposeful fetal movements. Kicks, rolls, hiccups. You'll probably start noticing daily rhythms — busier and quieter times — and partners can often feel movements clearly with a hand on the bump.
• Heartburn and indigestion — often more noticeable now as your uterus presses on your stomach. Small, frequent meals help; try not to lie down right after eating.
• Back and hip pain. Ligaments loosened by pregnancy hormones plus a shifting center of gravity make this common. Prenatal stretching, gentle movement, and a supportive pillow at night all genuinely help.
• Itchy skin over the stretching belly and breasts. Gentle moisturizer is usually enough. Widespread, intense itching — particularly on hands or feet — should always be reported to your clinician.
• Mild swelling in feet, ankles or hands, especially at the end of the day. Elevating your feet and staying hydrated help. Sudden or dramatic swelling deserves a call.
• Hemorrhoids. Not glamorous, unfortunately common. Fiber, fluids and avoiding straining help; ask your clinician about pregnancy-safe relief if they're bothering you.
• Trouble sleeping. Bump-related awkwardness, needing the bathroom, and racing pregnancy-brain thoughts all conspire. Side-sleeping with pillows, a wind-down routine, and gentle daytime movement all help.
• Braxton Hicks contractions. Brief, painless tightening. They should not become regular or painful — if they do, contact your clinician.

The always-call flags are the same as they've been: severe or persistent headache, sudden swelling of the face or hands, changes in vision, upper-abdominal pain, painful or regular contractions, any fluid leaking, any bleeding, or a noticeable decrease in your baby's movement. Trust your instincts — if something feels off, call.

Nutrition tips for week 25

Steady, balanced eating remains the goal. Nothing radical changes at 25 weeks — you're just supporting continued baby growth (especially lungs and brain) and your own energy. A few emphases:

• Steady blood sugar. If your glucose screening is coming up or has just happened, this is a good week to notice which meals leave you steady and which spike-and-crash you. Balanced plates — protein, fiber, healthy fats, quality carbs — win.
• Iron. Blood volume is still increasing. Lean meats, lentils, beans, tofu, spinach, fortified cereals; pair plant sources with vitamin C.
• Calcium and vitamin D. Baby's bones are hardening. Dairy, fortified plant milks, leafy greens, tinned fish with soft bones, and moderate sun exposure.
• Omega-3s (DHA). Continuing brain and eye development. Low-mercury oily fish twice a week, or a clinician-approved algae supplement.
• Fiber and fluids. Constipation and hemorrhoids don't just resolve themselves. Whole grains, fruit, vegetables, and roughly 8–10 glasses of water a day.
• Small, frequent meals. Kinder to your stomach with the growing uterus pressing on it, and steadier for energy.

The usual avoid list still applies: unpasteurized dairy, undercooked meat and eggs, high-mercury fish, deli meats you haven't heated through, and alcohol. Caffeine in moderation (per your clinician's guidance) is generally considered fine.

Safe exercise this week

For most healthy pregnancies, moderate movement remains genuinely good for you — better sleep, mood, back comfort, glucose regulation, and labor recovery. Aim for something most days; conversation-pace is a good intensity check.

Good options at 25 weeks:

• Walking — reliable and forgiving.
• Prenatal yoga — particularly good for hip openers, back pain relief and breath awareness.
• Swimming or aqua aerobics — the water takes weight off aching joints and feels amazing.
• Stationary cycling — low fall risk, adjustable intensity.
• Light strength or resistance work — focus on posture, core stability and pelvic floor.

Skip or adapt: anything with fall or contact risk, lying flat on your back for long periods, hot yoga, and pushing yourself out of breath. Stop and check in with your clinician if you feel dizzy, short of breath, have chest pain, painful or regular contractions, unusual pelvic pressure, fluid leaking, or any bleeding.

Questions to ask your doctor

You may have a check-up around now, and if you don't, your next one is close. Good questions to bring:

• Has my glucose screening been scheduled — and are the results in yet?
• Are baby's growth measurements on track?
• What preterm labor signs should I watch for from here?
• When should I start counting kicks daily, and what pattern is normal for my baby?
• Is my blood pressure and iron level where you'd like it?
• When is my next vaccination — for example the recommended tetanus/pertussis (Tdap) shot?
• When can I book a hospital tour and start sketching a birth plan?

If you'd like the wider context, our companion guides for pregnancy week 23 and pregnancy week 24 cover the weeks either side. Our simple due date calculator is handy for the big picture, and if you know someone at the very beginning of trying, two-week wait symptoms is a gentle read for those earliest days.

3 practical tips for week 25

1. Start noticing your baby's rhythm. Every baby has quieter and busier times. Getting familiar with yours makes it far easier to notice, later, if anything changes — and gives you a lovely sense of connection in the meantime.
2. Book a hospital tour, or research one online. Knowing where you'll go, how you'll get there, and what the space feels like removes a surprising amount of third-trimester anxiety. And most tours book up faster than you'd expect.
3. Sort the practical prep in small pieces. Don't try to tackle everything at once. Ten minutes on a nursery list, or fifteen on a shortlist of names, or a quick chat about parental leave — steady, small, done.

How Vyve helps you track pregnancy — privately

Pregnancy is enormous, personal and fast-moving. Between kick counts, symptom notes, weight, mood, appointments and everything you meant to ask the doctor, it's easy to lose the thread. That's the gap Vyve is built to fill — a private AI pregnancy tracker that lives on your phone, learns your patterns, and turns months of scattered notes into a clean picture you can actually use.

Vyve treats your pregnancy as the source of truth — not a generic 40-week template. It tracks how your symptoms cluster, when kicks are strong, how your sleep and mood shift week to week, and exports a doctor-ready summary you can bring to appointments. And it does it privacy-first: the AI runs on your device, your data is encrypted and stays on your phone, there's no required account, and nothing about your pregnancy is sold or shared. For something this intimate, that matters.

Frequently asked questions

How big is my baby at 25 weeks pregnant?

At 25 weeks, your baby is roughly the size of a cauliflower or rutabaga — about 34 centimeters (around 13.5 inches) from head to heel and weighing approximately 660 to 700 grams (about 1.5 pounds). Baby is lengthening steadily and starting to plump up as fat continues to build under the skin.

What is happening with my baby's lungs at 25 weeks?

At 25 weeks, your baby's lungs are in a critical stage of development. Tiny air sacs called alveoli are multiplying, and specialized cells are producing surfactant — a substance that keeps the air sacs from collapsing after birth. Blood vessels around the lungs are also proliferating. Lungs aren't fully ready to breathe air on their own yet, but the essential architecture is developing rapidly this week.

What symptoms are normal at 25 weeks pregnant?

Common week 25 symptoms include a bigger, higher bump, strong fetal movements, heartburn, back and hip pain, mild swelling, hemorrhoids, occasional trouble sleeping, and itchy skin over the belly. Most are normal second-trimester experiences — but severe headaches, sudden swelling, changes in vision, upper-abdominal pain, or reduced fetal movement should always be checked with your obstetrician.

Is it safe to exercise at 25 weeks?

For most healthy pregnancies, yes — moderate exercise remains beneficial. Walking, prenatal yoga, swimming, stationary cycling and light strength work are widely recommended. Avoid lying flat on your back for long periods, contact sports, and anything with a fall risk. Your obstetrician or midwife is the best person to confirm what's right for your specific pregnancy.

What should I ask my doctor at 25 weeks?

Useful questions at 25 weeks include: has my glucose screening been scheduled, are baby's growth and my blood pressure on track, what preterm labor signs should I watch for, when should I start counting kicks daily, and when is my next tetanus/pertussis or other recommended vaccination? This is also a great time to book a hospital tour and start sketching out a birth plan.