Baby Development Week by Week
From the first heartbeat at week 6 to a full-term baby at week 39, every week brings new milestones. Here's what's happening inside the womb at each stage, and the major scan milestones that matter most along the way.
The Six Major Milestones
Six weeks of pregnancy mark genuine turning points in baby's development, and in what scans become useful. Each links to a deep guide with full clinical detail.
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
Pregnancy is divided into three trimesters, each lasting roughly 13 weeks. The first trimester (weeks 1–13) is the period of greatest structural change, when every major organ system is built from scratch in just a few months. The second trimester (weeks 14–26) is when those systems refine, baby grows recognisable, and movement becomes felt. The third trimester (weeks 27–40) is when growth and brain development accelerate, lungs mature, and baby prepares for life outside the womb.
The timeline below covers every key week of pregnancy with what's happening biologically and which scans are most relevant. Six weeks are highlighted as major milestones: these are the gestations where what's possible on ultrasound shifts meaningfully, where private scans tend to be most useful, and where the NHS pathway has its key checkpoints. The other weeks aren't unimportant; they're just less of a turning point.
If you're not sure which scan to book at your current week, the milestone cards above are the fastest way to find your bearings. For broader context, our Private vs NHS comparison guide explains how the two pathways complement each other.
Foundation systems form
The rudiments of the respiratory, digestive, circulatory, nervous and excretory systems are formed. The heartbeat starts.
Major organs begin
The rudiments of the intestine, liver, kidney and lung are formed. The muscular system and the spine develop. The neural tube completes its formation.
Organs and limbs
Organs and systems start to form, as do parts of the brain. The umbilical cord appears. The upper lip and nasal cavity are formed, alongside the rudiments of the extremities and nails.
Brain divisions + first heartbeat visible
The brain and its divisions form. The placenta begins, the muscles of the face start to develop, and the heartbeat becomes visible on ultrasound for the first time. Six weeks is the earliest reliable private reassurance scan window.
6 Week Scan : full milestone guide →Uteroplacental circulation begins
Uteroplacental circulation starts. The upper limbs develop. The eyes and the rudiments of the ears begin to form.
Book a 7-week reassurance scan →Major systems complete + DNA gender test window opens
The heart, lungs, brain, urinary system and reproductive system are formed. Eyes, eyelids, nose, ears and the formation of the lips are completed. From this week, sufficient fetal DNA circulates in the mother's bloodstream for the Gender Blood Test.
Gender Blood Test available from 8 weeks →NIPT screening window opens
Sufficient fetal DNA now circulates in the mother's bloodstream for non-invasive prenatal testing (NIPT). Major organs are formed and now refining.
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. Crown-rump length is measured for accurate dating. The NHS dating scan is scheduled this week, often combined with nuchal translucency screening. A private 12-week scan complements the NHS appointment with longer time and family welcome.
12 Week Scan : full milestone guide →First hair + bone development
First hair appears and bones become more robust. Changes take place in the genital organs: the prostate gland forms in boys, and ovaries migrate to the pelvic cavity in girls.
Gender visible on ultrasound
Anatomy has now developed enough that gender can be reliably identified on a 2D ultrasound scan with 99%+ accuracy.
Gender Scan from 15 weeks →Gender determination sweet spot
Baby is roughly the size of an avocado. The genital tubercle has fully differentiated, gender is unambiguous on ultrasound, and there's still room for baby to be in a cooperative position. The single best week for visual gender confirmation. Many mothers may begin to feel the first flutters of movement.
16 Week Scan : full milestone guide →Senses + immune system
The immune system is created. Modes of sleep and wakefulness begin to take effect. Baby has the ability to distinguish sounds and respond to the surrounding world.
NHS anomaly scan + halfway mark
Baby is around 25cm long, weighing 300g. All major organs are fully developed. The NHS schedules its anomaly scan this week, the diagnostic gold standard for screening 11 specific fetal conditions. Most mothers feel definite movement by now. A private 20-week scan complements (rather than replaces) the NHS appointment.
20 Week Scan : full milestone guide →Face fully formed for 4D
Bone marrow and spleen start to produce blood cells. Taste buds develop. The face is fully formed, including eyelashes and eyebrows, ideal for 4D HD Live scans.
4D Baby Scan from 24 weeks →Growth measurements begin
Baby gains rapidly in weight and length. This is when growth and presentation scans become most useful.
Growth & Presentation Scan from 24 weeks →Third trimester begins + 4D bonding sweet spot
Baby is around 38cm long and weighs roughly 1kg. Eyes open for the first time. The face has filled out with subcutaneous fat for soft, photorealistic features. The single best week of the 24–34 week 4D HD Live window, when baby has filled out beautifully and there's still plenty of room to move.
28 Week Scan : full milestone guide →Skin smooths, hormones begin
The folds in the skin start to disappear. Hands and feet become more plump. The skin ceases to be red. The endocrine system begins to secrete hormones. The lungs accumulate surfactant in preparation for breathing.
Position settles + birth preparation
Baby is around 47cm long and 2.6kg, close to birth weight. Subcutaneous fat continues to develop. Most babies have settled into birth position (typically head-down). The lungs are producing surfactant in adequate quantities. The skull bones remain soft with fontanelles to allow for birth. The right week for a private growth and presentation scan.
36 Week Scan : full milestone guide →Full term reached
Baby is now considered full-term. All organs are mature and ready for life outside the womb. The average baby weighs around 3.4 kg (7.5 lbs) at this stage.
Full Guides for Each Major Milestone
Each of the six major milestone weeks has its own deep guide: what's happening biologically, what the scan shows, how it compares to NHS care, and which packages 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 →Bond With Your Baby At Every Stage
From early reassurance to 4D bonding, our award-winning Gateshead clinic offers scans for every week of your pregnancy. CQC-regulated, experienced and registered sonographers, a fast-growing wall of five-star Google reviews since 2025.