# Human prenatal anatomy: modeling reference

Researched 24 October 2026. This is guidance for an original educational illustration, not a scan-derived atlas, diagnosis, measured lineage, or simulation of one individual.

## Dating and scope

PF = time after fertilization. GA = gestational age counted from the last menstrual period; GA is approximately PF + 2 weeks. The difference varies in real pregnancies. Use PF days for the early stops and explicitly labeled pregnancy weeks later. The embryo/fetus boundary follows about 8 completed PF weeks (approximately 10 GA weeks). Carnegie stages describe morphology, not exact birthdays, and end at CS23. Do not label fetal stops with Carnegie or mouse Theiler stages. These 16 stops are representative snapshots, not equal time intervals. [Carnegie staging](https://embryology.med.unsw.edu.au/embryology/index.php/Carnegie_Stages), [human timeline and variability](https://embryology.med.unsw.edu.au/embryology/index.php/Timeline_human_development).

## Stage matrix

Geometry instructions below are design interpretations of the cited anatomy, rather than measurements extracted from those sources. Approximate dates are anchors; each model should be labeled illustrative.

| Stop | Age / label | Shape and structures to reveal | Change / absent structures | Basis |
|---|---|---|---|---|
| 1 | PF day 1 · Fertilization | One rounded cell within a zona pellucida; a restrained internal pronuclear depiction if shown. | No tissue layers, organs, amnion, limbs, or cord. | [Week 1](https://embryology.med.unsw.edu.au/embryology/index.php/Week_1) |
| 2 | PF day 3 · Cleavage | A compact cluster of blastomeres within approximately the same overall zona envelope. | More cells without a correspondingly enlarged conceptus; no blastocoel yet in a morula depiction. | [Carnegie collection](https://medicalmuseum.health.mil/assets/documents/collections/hdac/developmental_stages_in_human_embryoes.pdf) |
| 3 | PF day 5 · Blastocyst | Hollow outer trophectoderm shell, fluid cavity, inner cell mass clustered at one pole. | Large cavity replaces solid cluster; zona thinning/hatching may be shown. No miniature body inside. | [Week 1](https://embryology.med.unsw.edu.au/embryology/index.php/Week_1) |
| 4 | PF day 10 · Implantation | Small bilaminar disc between amniotic cavity and yolk-sac compartment; surrounding trophoblast and optional uterine contact. | Flattened disc, not an embryo-shaped silhouette. Distinguish embryonic from supporting tissues. | [Implantation teaching material](https://embryology.med.unsw.edu.au/embryology/images/6/65/BGDAPracManual_2013-Prac6.pdf) |
| 5 | PF day 17 · Germ layers | Trilaminar disc with ectoderm, mesoderm, endoderm; primitive streak/notochordal region as a subtle midline cue. | Three layers replace bilaminar organization. No mature heart, lungs, limbs, or skeleton. | [Human timeline](https://embryology.med.unsw.edu.au/embryology/index.php/Timeline_human_development) |
| 6 | PF day 24 · Body folding | C-shaped axis, partially closing neural tube, cranial expansions, paired somites, prominent looping heart and primitive gut. | Disc becomes a folded body; avoid four developed limb buds at this anchor. Neuropore closure remains in progress. | [CS10–12 descriptions](https://medicalmuseum.health.mil/assets/documents/collections/hdac/developmental_stages_in_human_embryoes.pdf), [neural development](https://embryology.med.unsw.edu.au/embryology/index.php/Neural_System_Development) |
| 7 | PF day 35 · Early organs | Large cranial vesicles, optic structures, pharyngeal contours, heart/liver bulges, short tail, four limb buds with early hand plates; small branching respiratory buds. | Distinct organ primordia replace simple tubes. No separated fingers or adult brain grooves. | [CS14–15 descriptions](https://medicalmuseum.health.mil/assets/documents/collections/hdac/developmental_stages_in_human_embryoes.pdf), [lung development](https://embryology.med.unsw.edu.au/embryology/index.php/Respiratory_System_Development) |
| 8 | PF day 56 · Late embryo | Rounded large head, elongating trunk, elbows, fingers/toes, regressed external tail; chambered heart, growing liver, branched lungs, herniated intestinal loops. | Human limb configuration is recognizable. Do not leave a long tail or replace developing skeleton with a complete adult skeleton. | [CS23](https://medicalmuseum.health.mil/assets/documents/collections/hdac/developmental_stages_in_human_embryoes.pdf), [human intestinal reconstruction](https://pubmed.ncbi.nlm.nih.gov/26599074/), [limbs](https://embryology.med.unsw.edu.au/embryology/index.php/K12_Limbs) |
| 9 | Pregnancy week 12 · Early fetus | Large head relative to body, defined limbs and digits, closed eyelids; gut returns to abdomen, branching lungs, fetal heart, prominent liver. | External herniated bowel is removed as return completes; no external tail. | [Intestinal return study](https://pmc.ncbi.nlm.nih.gov/articles/PMC6422813/), [fetal overview](https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development) |
| 10 | Pregnancy week 16 · Growing limbs | Lengthened trunk/legs, more defined neck and face, thin skin; cartilage with selected ossification centers. | Head becomes a smaller fraction of whole length. Eyelids remain closed. | [ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy), [limb development](https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Limb_Development) |
| 11 | Pregnancy week 20 · Changing proportions | Slender limbs, recognizable facial features, growing brain and cerebellum; optional fine lanugo/vernix treatment. | Distinct fetal proportions rather than a scaled week-12 body. Brain surface should remain comparatively simple. | [Fetal development](https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development), [vernix](https://embryology.med.unsw.edu.au/embryology/index.php/Integumentary_System_Development_-_Vernix_Caseosa) |
| 12 | Pregnancy week 24 · Developing detail | Slender body, soft skin folds, relatively little subcutaneous fat; increasingly elaborated brain surface and lung tissue. | Do not render the plump body or deeply folded adult brain of a later stage. | [ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy), [cortical folding research](https://pubmed.ncbi.nlm.nih.gov/38979235/) |
| 13 | Pregnancy week 28 · Maturation | Fuller trunk/limbs, eyelids may open, stronger cortical surface features; more developed peripheral lung compartments. | Skin becomes less translucent; brain geometry gains structure. Lungs remain prenatal, fluid-filled. | [ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy), [respiratory development](https://embryology.med.unsw.edu.au/embryology/index.php/Respiratory_System_Development) |
| 14 | Pregnancy week 32 · Filling out | More rounded cheeks, abdomen and limbs; greater brain surface complexity, growing organs, increasing skeletal mineralization. | Add volume selectively to fat-bearing regions, not uniform scaling. | [Fetal development](https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development), [cortical folding research](https://pubmed.ncbi.nlm.nih.gov/38979235/) |
| 15 | Pregnancy week 36 · Late fetus | Fuller limbs, smooth skin, recognizable fingers/toes and nails; flexed posture may become more compact. | Do not imply skull sutures have fused or that all fetuses adopt one pose. | [ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy), [fetal development](https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development) |
| 16 | Pregnancy week 40 · Before birth | Near-term proportions, rounded cheeks/limbs, folded pose, umbilical connection; fetal organs remain inside the intact body. | No first breath, cord clamping, postnatal circulation, or assertion that brain/lung development is complete. | [ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy), [heart development](https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Heart_Development) |

## System transformations to encode

- **Neural:** early sheet → open groove/tube → cranial vesicles and spinal cord → expanding hemispheres and cerebellum → increasingly folded cerebral surface. Avoid keeping a row of neural-tube beads in a near-term fetus. Surface folding varies regionally; regular sinusoidal corrugation is only an illustrative texture, not a map of named sulci. The MRI research covers second-to-third-trimester change rather than proving a uniform folding rate. [Neural development](https://embryology.med.unsw.edu.au/embryology/index.php/Neural_System_Development), [primary cortical MRI research](https://pubmed.ncbi.nlm.nih.gov/38979235/).
- **Heart:** paired precursors → fused/looping tube → changing chambers/outflow → fetal four-chamber configuration. Early heartbeat or schematic blood flow does not imply adult circulation. A near-term view must preserve fetal shunts rather than close them before delivery. [Heart development](https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Heart_Development), [ductus venosus](https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Ductus_Venosus).
- **Gut:** primitive tube → fore/mid/hindgut and growing organ buds → elongation and physiological herniation → return and increasingly packed abdominal loops. A primary MRI study finds herniation apparent after CS17 and increasing complexity thereafter; use it rather than an arbitrary identical coil. Do not animate rigid uniform rotation as a measured mechanism. [Ueda et al.](https://pubmed.ncbi.nlm.nih.gov/26599074/), [early fetal intestinal return](https://pmc.ncbi.nlm.nih.gov/articles/PMC6422813/).
- **Lungs:** a foregut outgrowth and paired buds precede branched conducting airways; later internal organization expands into future gas-exchange regions. The histological stages overlap. Do not label a macro 3D mesh as a measured alveolar reconstruction; do not show air inflation in utero. Lung development continues after birth. [UNSW respiratory development](https://embryology.med.unsw.edu.au/embryology/index.php/Respiratory_System_Development).
- **Skeleton/limbs:** early somites are transient segments and contributors, not permanent beads on a fetus. Show limb buds → plates/digital rays → separated digits and joints. Cartilage precedes substantial ossification; use separate appearance or geometry for mineralization rather than a fully white adult skeleton from the beginning. [Limb teaching reference](https://embryology.med.unsw.edu.au/embryology/index.php/K12_Limbs), [limb development lecture](https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Limb_Development).
- **Support:** amniotic space, yolk-sac compartment and trophoblastic/placental tissue have different identities. A simple cord/placenta context is appropriate later; a gigantic persistent yolk sac is not. Keep external support optional so internal organization remains readable. [Placenta development](https://embryology.med.unsw.edu.au/embryology/index.php?title=Lecture_-_Placenta_Development).

## Geometry and interaction recommendations (design judgments)

1. Use separate model families for cell/cluster, cavity/disc, folding embryo, limb-bearing embryo, and fetus. Interpolate only within compatible families; crossfade incompatible topology and label transitions illustrative.
2. Early explode groups: zona/cell; blastomeres; trophectoderm/inner cell mass; epiblast/hypoblast/support; ectoderm/mesoderm/endoderm. Later explode groups: outer form, nervous system, heart/vessels, gut/liver, respiratory system, skeleton, optional support.
3. After week 12 many groups legitimately recur. Their relative shape, placement, branching, surface maturity and relationship to the body must change. Do not invent organ appearance/disappearance simply to make controls vary.
4. Define stage-specific part names and descriptions. Hide a selected part if absent, or map it to an explicitly related successor; never silently claim exact cell lineage.
5. Blend limb roots into torso, digits into palms and facial structures into cranium. Fetal head should progressively occupy less of total length while cheeks and limb fat increase later. Inspect silhouettes from multiple viewpoints.
6. Auto-framing hides growth. Label the viewport as independently framed and provide a scale cue only when the geometry has a defensible scale. A made-up exact length is worse than no number.

## Limits to publish in Sources/About

Original schematic human anatomy, based on published developmental descriptions. Not a patient scan, whole-body measured atlas, predictive model, or diagnostic tool. Dates approximate; individual development varies. Some organs and microanatomy omitted. Colors, transparency, exploded spacing, poses and crossfades are presentation choices. Cell-level surface particles, if used, are not measured cells. No claims of validated anatomical accuracy without an expert review.

## Source caveats

UNSW mixes PF and GA tables on some pages, especially lung-stage summaries. Read each caption before assigning numeric boundaries. ACOG is a useful accessible overview, but some bullets compress long processes; avoid translating phrases such as organs being “done” into claims that development ends at birth. Original human specimen/MRI studies are the stronger basis for detailed shapes. Publicly viewable figures are not automatically reusable assets; no source imagery or mesh reuse is assumed here.

## Guided journey and support visibility

The 60-second sequence uses 16 stages and 18 explanations. Each caption is 10–18 words with at least three seconds of display time. Captions describe the selected stage or visible structure; stage progression, separation, and growth transitions remain illustrative. The cord and placenta appear by default from the first modeled support stage. The cord is anchored to each body mesh; its route and the small placental disc are presentation choices rather than measured anatomy. Placental support and prenatal circulation are described using [UNSW Placenta Development](https://embryology.med.unsw.edu.au/embryology/index.php/Placenta_Development), alongside the stage references above.