Week-by-Week Pregnancy Guide: Weeks 4-40

In brief: pregnancy lasts around 40 weeks from the date of your last period and is divided into three trimesters. In the first trimester (weeks 4-13) all your baby's essential organs take shape and the heart starts beating; in the second trimester (weeks 14-27) you feel the first movements and see your baby at the detailed scan; in the third trimester (weeks 28-40) your baby gains weight rapidly, the lungs mature and they prepare for birth.

Pregnancy is an extraordinary journey in which an embryo the size of a lentil becomes, over 40 weeks, a baby you hold in your arms. Every week brings a new development and every trimester a new period. This guide takes pregnancy week by week across the three trimesters, covering how much your baby has grown at each stage, the milestones they pass and what's changing in your body.

If you aren't sure how many weeks pregnant you are, have a look first at our article on how to calculate your pregnancy week. A quick reminder: pregnancy is counted from the first day of your last period, not from conception, which is why you're usually already 4-5 weeks along when the test turns positive.

First Trimester (Weeks 4-13): Laying the Foundations

The first trimester is the least "visible" from the outside but the most intense biologically. Your bump may not show yet, but inside, a new structure appears every day: your baby's heart, brain, spinal cord, internal organs and limbs all begin taking shape in these weeks. That's why folic acid and balanced eating matter most in this period — see our pregnancy nutrition guide for the details.

Weeks 4-7: From Poppy Seed to Lentil

At week 4 the embryo is the size of a poppy seed and has only just implanted into the wall of the uterus. By week 5 the neural tube — the foundation of the brain and spinal cord — begins to close. The most exciting development is the heart: the heartbeat generally starts at 5-6 weeks and can be seen as a flicker on an early ultrasound. At 6-7 weeks the embryo reaches the size of a lentil; arm and leg buds appear and the first outlines of the face begin to form.

Weeks 8-10: A Fetus the Size of a Grape

At week 8 the embryo is roughly the size of a grape. Fingers begin to separate, eyelids and outer ears take shape, and the arms can bend at the elbow. Your baby has in fact already started moving in these weeks — but they're still far too small for you to feel it. Towards the end of week 10 the medical term changes: no longer an "embryo" but a "fetus". That means the basic outlines of every organ are in place. The weeks that follow are weeks of growth and maturation.

Weeks 11-13: Towards Lemon Size — 12 Weeks Pregnant

Twelve weeks is a symbolic threshold for many mothers-to-be: the first-trimester screening test (nuchal translucency measurement and blood tests) is generally done between weeks 11 and 14, and the risk of miscarriage drops markedly after this point. Your baby is now around the size of a lemon; nails appear on the fingertips and the kidneys begin working and producing urine. Many couples choose to share the news with those around them in these weeks.

What does the mother experience? The classic trio of the first trimester is nausea, fatigue and emotional swings. Because of the rapid rise in hormones, morning sickness (which can in fact strike at any hour), sensitivity to smell, cravings or aversions to certain foods, breast tenderness and frequent urination are all common. These symptoms vary enormously from person to person; having no nausea at all is entirely normal too. If nausea is seriously stopping you eating, do consult your doctor.

Second Trimester (Weeks 14-27): the Honeymoon Period

The second trimester is remembered by most mothers-to-be as the most enjoyable. Nausea generally recedes, energy returns and the bump starts to show — pregnancy becomes "visible" to you and to those around you. On your baby's side, these are the weeks when the outlines of organs turn into working systems, the senses awaken and growth visibly accelerates.

Weeks 14-17: A Baby the Size of an Avocado

The second trimester officially begins with week 14. At week 16 your baby is around the size of an avocado. Fine hair called lanugo starts covering the body and the facial muscles work: your baby can frown, swallow and make sucking movements. The skeleton continues turning from cartilage into bone; the legs are now longer than the arms and the joints work actively. Fingerprints also form in this period — the patterns that make your baby unique in the world are ready long before you hold them.

Weeks 18-22: Mango Size, First Kicks and the Anatomy Scan

These weeks are the star of the second trimester. At weeks 19-20 your baby reaches roughly the size of a mango. Feeling the first movements (quickening) generally happens between weeks 18 and 22; it can be noticed a little later in first pregnancies and earlier in subsequent ones. The faint flutters first mistaken for wind turn into distinct kicks as the weeks pass.

The detailed (second-level) anatomy scan carried out around 20 weeks is the most comprehensive check, examining your baby's organs one by one; if you want to know, the sex usually becomes clear at this scan too. The white, creamy vernix layer that protects your baby's skin from the amniotic fluid also starts forming in this period.

Weeks 23-27: From Corn Cob to Cauliflower — the Viability Threshold

At week 24 your baby is around the length of an ear of corn, and a medically important threshold is crossed: week 24 is regarded as the limit of viability, and the chance of survival with intensive care support rises markedly for babies born after this week. The lungs begin producing surfactant, the substance that makes breathing possible — though the lungs continue maturing throughout the third trimester. Your baby's hearing has developed; they start recognising your voice and may respond to loud sounds with movement. By week 27 your baby approaches the size of a cauliflower, the eyelids open and a sleep-wake pattern becomes established. Gestational diabetes screening (the glucose tolerance test) is generally done between weeks 24 and 28.

What does the mother experience? With energy returning and appetite picking up, weight gain accelerates. Sharp twinges in the groin from the stretching ligaments of the growing uterus (round ligament pain), a blocked nose, skin darkening and stretch marks on the abdomen can all appear. Regular balanced eating and gentle exercise with your doctor's approval are the best investments of this period.

Third Trimester (Weeks 28-40): Preparing for Birth

The final trimester is your baby's "finishing school": they gain weight, their lungs mature and they move into position for birth. A significant share of your baby's birth weight is gained in these last weeks. For you it's both the most physically demanding period and the one where the excitement peaks; the hospital bag, the nursery and the birth plan are all on the agenda in these weeks.

Weeks 28-31: Rapid Growth Begins

The third trimester starts at week 28; your baby is around the size of an aubergine and can open and close their eyes. Brain development gains tremendous speed in this period; the folds on the surface of the brain become defined and billions of neural connections form. Your baby now has regular sleep cycles and responds to light and sound; you may feel their hiccups as rhythmic twitches. As space inside the uterus gets tighter, the character of the movements changes: wide somersaults give way to pushes, stretches and kicks. The shape of the movements may change but the pattern should hold; getting to know your baby's daily movement rhythm is one of the most valuable habits of this period. Appointments also start becoming more frequent from here.

Weeks 32-35: Melon Size and Head Down

At weeks 32-34 your baby reaches roughly the size of a melon, and most babies turn head down in these weeks. The bones harden (the skull stays flexible to ease the passage through the birth canal), fat tissue builds up under the skin and your baby gradually becomes rounder. Surfactant production in the lungs speeds up; the likelihood of breathing problems drops markedly for babies born after week 34.

Weeks 36-40: Watermelon Sized — It Could Be Any Time

In the final weeks your baby approaches the weight and size of a watermelon; average birth weight is around 3-3.5 kg. From week 37 pregnancy is considered "term", and weeks 39-40 are defined as "full term". Your baby's head settles into the birth canal (engagement) — you may feel this as your bump dropping and your breathing easing. The lanugo largely sheds and the lungs are ready for birth. All that's left is waiting: only a small share of births happen exactly on the estimated date, and going a few days past week 40 is entirely ordinary.

What does the mother experience? Because the growing uterus presses on the diaphragm, breathlessness is common, along with reflux and indigestion from pressure on the stomach, back pain, swollen legs and sleep problems. Braxton Hicks contractions — irregular, painless practice tightenings — become noticeable in this period; contractions that become regular, increasingly frequent and stronger are the sign of real labour and the point at which to call your doctor. If your waters break, you have bleeding, or you notice a clear reduction in your baby's movements, go to a health facility without delay.

A Checklist for Each Trimester

Trimester	Highlights	Recommendations for the mother
1st trimester (weeks 4-13)	Heartbeat begins (weeks 5-6), organs form, screening test at weeks 11-14	Folic acid and the vitamins your doctor recommends, first appointment, stopping smoking and alcohol, plenty of rest
2nd trimester (weeks 14-27)	First movements (weeks 18-22), anatomy scan (weeks 18-22), viability threshold (week 24), glucose screening (weeks 24-28)	Balanced eating and controlled weight gain, exercise with your doctor's approval, looking into antenatal classes
3rd trimester (weeks 28-40)	Rapid weight gain, lung maturation, turning head down (weeks 32-36), term period (week 37+)	Tracking baby movements, preparing the hospital bag and birth plan, learning the signs of labour, keeping every appointment

For more detail on the official definitions of the trimesters and the term/preterm concepts, see our pregnancy calculation article.

Make Week-by-Week Tracking Easier

Across this forty-week journey it isn't easy to hold "how much has my baby grown this week, which check was due?" in your head. Baby Wallie's pregnancy tracking mode calculates your pregnancy week automatically once you enter the date of your last period and brings your baby's development, their size and information specific to that week in front of you every week. You can record your blood pressure, weight and symptoms in the same place.

Wallie AI takes on the personalised side: the AI assistant builds weekly nutrition, sleep, breathing and Kegel programmes around your pregnancy week, and as birth approaches it prepares a birth plan with you. You can put general questions about your week to Wallie AI at any hour. Wallie AI provides information and doesn't replace an examination: if a symptom worries you, see your doctor. You'll find all our other pregnancy and baby care content on our guide page.

Frequently Asked Questions

I still can't feel my baby's movements — should I be worried? In a first pregnancy it's normal not to feel movements until weeks 20-22, and an anterior placenta can delay the sensation too. If you still feel no movement after week 24, consult your doctor.

Do I need an ultrasound every week? No. In an uncomplicated pregnancy, checks happen at the intervals your doctor sets; more frequent checks in the last trimester are normal. Agree your monitoring plan with your doctor.

How realistic are the fruit comparisons? They're approximate sizes used to make things memorable. Every baby grows at a different rate; what matters isn't a single week's measurement but the growth curve your doctor is tracking.

My symptoms don't exactly match what's described — is that normal? Yes. Every pregnancy — even different pregnancies in the same woman — progresses differently. The weeks and symptoms in this guide reflect general averages; never hesitate to discuss anything that worries you with your doctor.

Next Steps

• Two exercises you can practise regularly to prepare for birth: Kegel and breathing exercises in pregnancy
• What to eat and what to avoid, trimester by trimester: pregnancy nutrition guide
• Preparing for after the birth: baby feeding schedule by month and the baby vaccine schedule

To find out how many weeks along you are and your estimated due date in seconds, use our pregnancy calculator.