Fetal Development Stages: Pregnancy Growth Milestones for Your Baby in Utero
- Makayla McRorie
- Sep 26, 2025
- 4 min read
Fetal development is a remarkable, rapid process that transforms a single fertilized cell into a fully formed baby. Understanding key milestones helps expectant parents appreciate how their baby grows and changes week by week. This guide focuses on your baby in utero and its fetal development stages—what develops when, how size and function progress, and what each stage typically looks like. Read on for clear, stage-by-stage prenatal development insights and practical takeaways.
Want to learn how the stages of pregnancy affect the mother? We have an article on that, too!
What to Expect Developmentally by Trimester: A Summary
First trimester: foundation of organs and basic structures; highest rate of structural formation.
Second trimester: significant growth, movement, sensory development, and early viability milestones.
Third trimester: maturation of lungs and brain, fat accumulation, and preparation for birth.
Fetal Development Stage-by-Stage
Conception and Implantation (Weeks 1–2 post-LMP with fertilization occurring around Week 2)
What happens? The sperm fertilizes an egg forming a zygote, beginning cell division and implantation of the blastocyst into the uterine lining.
Is there a baby at this point? While no heartbeat is present yet, there is rapid cell division establishing the embryonic disc and the beginnings of the placenta and amniotic sac!
Embryonic Period (Weeks 3–8)
Organ foundations: Major systems begin—neural tube (future brain/spinal cord), heart tube forms and starts beating by about week 5–6. Limb buds, facial features, and primitive lungs develop.
Size: about the size of a kidney bean by week 8.
Key milestone: organogenesis—basic structures are laid down; this is a critical window for healthy development.
Early Fetal Period (Weeks 9–12)
Rapid growth: bones start to ossify; limbs more defined; fingers and toes separate.
Functional beginnings: facial muscles allow small movements; the baby may make small reflexive motions.
Size: measures roughly 2 inches by the end of week 12.
Key milestone: fetus looks recognizably human; early sex organs differentiate.
Second Trimester / Mid-Pregnancy (Weeks 13–26)
Sensory and movement development: active movements become stronger; mothers may begin to feel fetal kicks (often 16–22 weeks). Taste buds form; ears can hear external sounds; ocular muscles and eyelids develop.
Organ refinement: lungs produce surfactant late in this period; kidneys produce urine; digestive tract practices swallowing.
Size & viability: weight and size increase rapidly. By about 24 weeks, some babies show viability with advanced neonatal support, though maturation continues with viability rates increasing every week.
Key milestone: robust growth and increasing functional maturity.
Third Trimester / Final Growth and Maturation (Weeks 27–40)
Brain development: dramatic growth and cortical folding increase; neural networks strengthen.
Lungs and other organs mature: surfactant production increases, improving independent breathing capacity at term.
Fat deposition: subcutaneous fat accumulates for temperature regulation and energy reserves.
Positioning: many babies assume a head-down (vertex) position in preparation for birth. Depending on where you plan to deliver, there may be some restrictions or alternate procedures in place for babies that still present a head-up (breech) position by week 36.
Size: most full-term babies weigh between 5.5–8.8 lbs (2.5–4 kg) and measure ~19–21 inches.
Key milestone: organ systems reach readiness for life outside the womb; continued fine-tuning of neurologic and metabolic functions.
Common Prenatal Tests and What they Reveal about the Baby
First-trimester screen (ultrasound, bloodwork): dating, nuchal translucency, early anatomy check.
Noninvasive prenatal testing (NIPT): fetal DNA screening for certain chromosomal conditions, usually completed in the first trimester.
Anatomy scan (~18–22 weeks): detailed organ assessment, growth measurements, placental location, and sometimes fetal sex. (PS–If the baby is at a good angle, your ultrasound tech may be able to snap some 3D images of your sweet babe!)
Third-trimester growth scans and fetal monitoring: assess growth trajectory, amniotic fluid, and well-being.
Supporting Baby and Mama at Haven Women's Health and Birth Center
At Haven Women’s Health and Birth Center, we celebrate positive, empowering birth experiences and focus on nurturing both baby and mother throughout every stage of prenatal development. Our care emphasizes continuous support, evidence-informed prenatal education, and personalized birth planning that centers the baby’s growth and the family’s preferences.
Haven offers comprehensive prenatal care that honors the positive journey of pregnancy and prepares families for optimal outcomes for baby and mama. Schedule a tour to see our birthing center and learn how Haven supports nurturing care from prenatal visits through postpartum transition. We’d love to show you how we prioritize both baby’s development and compassionate support for parents!
FAQs
When can I first feel my baby move?
Most people feel initial fetal movements (quickening) between 16 and 22 weeks; first pregnancies may notice movement a bit later. Movement detection and sensitivity also depends on where the placenta and baby are positioned. If you’re carrying multiples, you might feel them move sooner!
When is the baby’s sex typically distinguishable on ultrasound?
Sex can often be seen on an anatomy scan around 18–22 weeks, depending on fetal position and clarity. For those anxious to find out early, there are a handful of gender tests you can take both through your provider and outside companies.
How do fetal growth charts and percentiles work?
Growth charts compare an individual baby’s size (head circumference, abdominal circumference, femur length) to population norms to assess growth trajectory and detect potential concerns.
What developmental milestones indicate fetal well-being in the third trimester?
Regular, strong movements, steady weight gain on ultrasound, appropriate amniotic fluid, and mature breathing patterns on testing all signal good fetal progress.
Are fetal movements a reliable sign of the baby’s health?
Yes—consistent movements are reassuring! If you detect a significant decrease in fetal movement, please promptly contact your care team for evaluation.
Can environmental factors affect fetal development?
Yes—maternal nutrition, smoking, alcohol, certain medications, infections, and environmental toxins can influence development. Discuss any exposures with your clinician!
When does the baby become viable outside the womb?
With modern neonatal care, some babies born around 24 weeks may survive, but outcomes improve substantially with each additional week in utero, and full-term (39–40 weeks) is best for development.
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