Baby Development Week by Week
From the first detectable heart activity around week 6 of pregnancy to a full-term baby at week 39, human development unfolds remarkably quickly. A string of small, specific turning points lie in between. This is what's happening inside the womb each week, written for parents by an established Newcastle scan clinic, with the scan milestones that genuinely matter flagged along the way.
The Six Major Milestones
Six weeks stand out as genuine turning points, for what's developing in the womb, and for what a scan can reveal. Each card opens a longer guide with the clinical detail behind the milestone.
First Heartbeat
Earliest reliable private reassurance
Dating Scan
End of first trimester, NHS dating week
Gender Sweet Spot
99%+ accurate gender determination
Anomaly Scan
Halfway mark, NHS anomaly week
4D Bonding
The 4D HD Live sweet spot opens
Birth Prep
Position, growth, fluid before birth
The forty weeks of pregnancy are split into three trimesters of roughly thirteen weeks each. The first trimester (weeks 1-13) does the heavy lifting structurally: every major organ system is sketched out from nothing inside three short months. The second trimester (weeks 14-26) refines what's there, baby starts to look like a baby, and you begin to feel movement. The third trimester (weeks 27-40) is mostly about growth, brain wiring, lung maturation and getting ready for the outside world.
What follows is a week-by-week walk through what's actually happening in the womb, paired with which scans become useful at which gestation. Six of those weeks are flagged as milestones: they're the gestations where ultrasound suddenly reveals something it couldn't a fortnight earlier, or where the NHS pathway has a fixed checkpoint. The other weeks aren't unimportant. They're just less of an inflection point.
Not sure which scan fits where you are right now? The milestone cards above are the fastest shortcut. For the bigger-picture question of how a private scan sits alongside your NHS care, our NHS vs private comparison walks through the differences. And if you're newer to private scanning entirely, the self-referral guide covers the booking side.
Foundation systems form
The earliest building blocks of the respiratory, digestive, circulatory, nervous and excretory systems are laid down this week. Cells divide at extraordinary speed. The heart muscle starts twitching, yes, this early, though no scan will pick it up yet.
Major organs begin
Tiny precursors to the intestine, liver, kidneys and lungs take shape. The muscular system and the spinal column begin developing in parallel, and the neural tube, the future brain and spinal cord, finishes closing. A pivotal week, biologically.
Organs and limbs
More organs and systems begin forming, including parts of the brain. The umbilical cord appears, linking baby to the placenta-to-be. The upper lip and nasal cavity emerge, and the buds that will become arms, legs and even nails are now visible at a microscopic level.
Brain divisions + first heartbeat visible
The brain divides into its early regions. The placenta is establishing itself, facial muscles begin developing, and the heartbeat finally becomes visible on ultrasound. Six weeks is genuinely the earliest moment a private reassurance scan can show meaningful detail. Anything sooner is usually too soon to see.
6 Week Scan: full milestone guide →Uteroplacental circulation begins
Blood flow between the uterus and placenta is now properly established. The upper limbs lengthen visibly. Eyes appear as dark spots, and the rudiments of the ears form on the sides of the head.
Book a 7-week reassurance scan →Major systems complete + DNA gender test window opens
Heart, lungs, brain, urinary system and reproductive organs are all formed in miniature. Eyes, eyelids, nose, ears and lips finish their early shaping. From seven weeks onward there's enough fetal DNA in mum's bloodstream to run an Early Gender Blood Test reliably, an option many of our Newcastle clients reach for when they want to know early.
Early Gender Blood Test available from 7 weeks →NIPT screening window opens
Fetal DNA in the mother's blood now reaches a level where non-invasive prenatal testing (NIPT) can screen accurately for Down's, Edwards' and Patau's syndromes. The major organs are all in place; the rest of pregnancy is largely refinement and growth.
NIPT Test from 10 weeks (from £265) →End of first trimester + NHS dating scan
Baby is roughly the size of a lime, fully formed in miniature, with recognisable head, body, arms and legs. The sonographer measures crown-rump length to date the pregnancy precisely. This is the week the NHS schedules its dating scan, often paired with nuchal translucency screening. Plenty of our Gosforth families pair the NHS appointment with a private 12-week scan for a longer, more relaxed look, and so the wider family can come too.
12 Week Scan: full milestone guide →First hair + bone development
Fine downy hair begins to appear and bones harden noticeably. Differences emerge in the reproductive organs: boys begin forming a prostate gland, girls' ovaries shift down into the pelvic cavity.
Gender visible on ultrasound
Anatomy has matured enough that an experienced sonographer can identify gender on a 2D scan with 99%+ accuracy, provided baby cooperates and shows the right view.
Gender Baby Scan from 15 weeks →Gender determination sweet spot
Baby is about the size of an avocado. The genital tubercle has fully differentiated, gender is unmistakable on ultrasound, and there's still enough room in the womb for baby to lie in a helpful position. In our experience this is the single best week to visit for visual gender confirmation. Many mums also start feeling the first faint flutters, the kicks that get stronger by the day.
16 Week Scan: full milestone guide →Senses + immune system
The immune system comes online. Patterns of sleep and wakefulness start to appear. Baby can distinguish sounds, your voice begins to register, and reacts to the world beyond the womb.
NHS anomaly scan + halfway mark
Baby is around 25cm long and weighs roughly 300g. Every major organ has finished forming. This is the week of the NHS anomaly scan, the diagnostic gold standard, screening for 11 specific fetal conditions. By now most mums feel clear, regular movements. A private 20-week scan sits alongside the NHS appointment rather than replacing it, handy if you want extra time, family present, or 4D imaging the NHS doesn't offer.
20 Week Scan: full milestone guide →Face fully formed for 4D
Bone marrow and spleen start producing blood cells. Taste buds emerge. The face is now fully formed, eyelashes and eyebrows included; and that's why 24 weeks marks the start of the window for HD Live 4D imaging.
4D Baby Scan from 24 weeks →Growth measurements begin
Weight and length climb quickly from now on. This is the gestation when a private growth and presentation scan starts to earn its keep.
Growth & Presentation Scan from 24 weeks →Third trimester begins + 4D bonding sweet spot
Baby is now around 38cm long and weighs about a kilo. Eyes open for the first time. Subcutaneous fat fills the face out: soft cheeks, defined lips, that almost-photo quality everyone wants from a 4D HD Live bonding scan. Of the full 24–34 week window, week 28 is the one we'd pick: baby is plump enough to look like themselves, with plenty of space still to move.
28 Week Scan: full milestone guide →Skin smooths, hormones begin
Skin folds smooth out as fat deposits build. Hands and feet plump up. The reddish hue of earlier weeks fades. Baby's endocrine system starts producing hormones, and the lungs begin storing surfactant, the stuff that lets them inflate properly at birth.
Position settles + birth preparation
Baby is about 47cm long and 2.6kg, within touching distance of birth weight. Fat continues to lay down. Most babies have rotated head-down by now. Lungs are making surfactant in adequate amounts. Skull bones stay soft, with fontanelles, to fit through the birth canal. This is the week for a private growth and presentation scan if you're feeling unsure about position or size.
36 Week Scan: full milestone guide →Full term reached
Baby is officially full-term. All organs are mature and ready for life on the outside. The average baby tips the scales at around 3.4kg (7.5 lbs) by this stage, though there's a wide healthy range either side of that figure.
Full Guides for Each Major Milestone
Every milestone week has its own dedicated page: biology, what's visible on the scan, how it dovetails with your NHS appointments, and the Numi Scan packages that fit.
First Heartbeat
Earliest reliable private reassurance
Read full guide →Dating Scan
End of first trimester, NHS dating week
Read full guide →Gender Sweet Spot
99%+ accurate gender determination
Read full guide →Anomaly Scan
Halfway mark, NHS anomaly week
Read full guide →4D Bonding
The 4D HD Live sweet spot opens
Read full guide →Birth Prep
Position, growth, fluid before birth
Read full guide →A Scan For Every Stage Of Your Pregnancy
Two years on Bakers Yard in Gosforth, part of the award-winning Numi Scan group, with a fast-growing wall of five-star Google reviews since 2024 and counting. Whatever week you're at, from a first reassurance scan to a 4D bonding session before birth, there's a package that fits. CQC-regulated. experienced and registered sonographers. NE3 1XD.