Maternal-Newborn Nursing · Pregnancy
Fetal Growth and Development
On this page 9 sections
In 30 seconds
In about 38 weeks from conception (40 weeks counted from the last menstrual period), a single fertilized cell becomes a newborn capable of independent life. That journey divides into three periods with very different characters. The Germinal period First ~2 weeks: cell division and implantation Full entry → (conception to about 2 weeks) is cell division and implantation. The Embryonic period Weeks 3–8: organ formation (organogenesis) Full entry → (about weeks 3–8) is construction: the heart begins to beat, the neural tube closes, and the foundations of every organ system take shape — which is why it is the period of greatest vulnerability to harmful exposures (teratogens). The Fetal period Week 9–birth: growth and functional maturation Full entry → (about week 9 to birth) is growth and refinement: structures already built get bigger, more complex, and more functional, and survival depends less on organ presence and more on organ maturity.
Two ideas organize the whole topic. First, timing determines effect: the same exposure (alcohol, infection, certain drugs) can produce very different outcomes depending on which structures are forming when it occurs. Second, the fetus runs on a life-support system built from the pregnant person's body: the Placenta The exchange organ between maternal and fetal circulations Full entry →, membranes, umbilical cord, and Amniotic fluid Fluid surrounding and protecting the fetus Full entry →, each with its own functions and failure modes. Nurses teach these milestones, interpret fetal assessment tools, and explain what "viability," "maturity," and "term" actually mean.
Why this matters
- Teratogen Any exposure that can harm fetal development Full entry → counseling depends on timing. Knowing when organs form lets nurses explain why the first trimester is the highest-risk window — and why "the baby is formed by week 8" is true about structures but not function.
- Assessment tools measure the milestones. Ultrasound, fundal height trends, fetal movement counting, and fetal heart rate assessment all make sense only against a timeline of normal development.
- Viability and maturity are not the same. A fetus can be viable (able to survive outside the uterus with support) before its lungs and brain are mature. Nurses must be precise with these words for families making hard decisions.
- The support system has its own anatomy. Placental problems, cord accidents, and amniotic fluid abnormalities are major reasons for fetal surveillance — knowing the normal system is the prerequisite for recognizing the abnormal.
- Parent education is milestone education. "When will I feel it move? When can we hear the heartbeat? What is that white coating?" — nurses answer these every day.
The college version
Core Concepts
From Zygote to Embryo to Fetus
- Germinal period (conception–~2 weeks): rapid cell division produces a ball of cells that implants in the endometrium; placental structures begin to form.
- Embryonic period (~weeks 3–8): organogenesis. The neural tube closes, the heart begins beating, and limb buds, facial features, and internal organs take shape. By the end, most major structures exist in rudimentary form. This is the classic teratogen window — the highest-risk time for structural birth defects.
- Fetal period (week 9–birth): growth and maturation. Movement becomes coordinated, the skeleton hardens, skin and fat develop, the lungs produce Surfactant Substance that keeps lung air sacs open Full entry → (the substance that keeps air sacs open), and the brain grows explosively. Viability — survival outside the uterus with intensive support — becomes possible in the middle of the second half of pregnancy in well-equipped settings, and outcomes improve steadily with maturity.
Dating and Timing Vocabulary
Gestational age Age counted from the last menstrual period Full entry → is conventionally counted from the last menstrual period (LMP) — about two weeks before conception — which is why a "40-week" pregnancy is really about 38 weeks of fetal age. Fetal (conceptual) age is the time since fertilization. Knowing which clock is being used prevents serious miscommunication about "weeks" in counseling and documentation.
The Life-Support System
- Placenta: the interface between the two circulations — it delivers oxygen and nutrients, removes wastes, and produces the hormones (hCG, hPL, estrogen, progesterone) that maintain the pregnancy and drive the maternal changes of Topic 1. It is an organ of exchange, not a shield: many substances, including alcohol and certain medications, cross it.
- Umbilical cord: two arteries carry deoxygenated blood away from the fetus; one vein returns oxygenated blood — the reverse of adult vessels. Wharton's jelly Protective cushioning around cord vessels Full entry → cushions the vessels.
- Amniotic fluid and membranes: the amnion and chorion form the sac; fluid cushions against injury, allows the movement needed for muscle and joint development, maintains temperature, and gives room to practice breathing movements. Too little or too much fluid can signal problems and is assessed by ultrasound.
- Fetal circulation: unique shunts (ductus venosus, foramen ovale, ductus arteriosus) route blood around the nonfunctional fetal lungs and liver. At birth, when breathing begins, these shunts close — a transition detailed in later chapters on newborn adaptation.
Milestones Families Ask About
Heart tones are audible with a Doppler in the first trimester; ultrasound visualizes developing structures, and a detailed anatomy scan in the second trimester surveys the organs. Quickening First felt fetal movements — first felt movements — typically occurs in the second trimester: later for first pregnancies, earlier for those who have been pregnant before. Late in pregnancy, fetal movement counting (a daily pattern the pregnant person tracks and reports) becomes a routine well-being tool, with specific instructions varying by provider and institution. Around birth the fetus usually assumes a head-down position (cephalic presentation); other presentations (e.g., breech) are identified on assessment and influence birth planning.
Growth, Maturity, and the Vocabulary of Timing
"Preterm," "term," and "postterm" describe where birth falls on the gestational timeline; gestational age says where the fetus is, while birth weight and size-for-gestational-age describe how growth went (small or large relative to peers). These distinctions matter enormously for newborn care planning and are developed in later chapters. Maturity means functional readiness of organs — most famously the lungs, where surfactant production determines the ability to breathe air. Fundal height (pubic bone to top of the uterus) rises in a fairly predictable pattern with gestational age and is a simple way the team tracks growth, with ultrasound as the more precise tool.
How It Works / Step-by-Step Process: Explaining a Fetal Assessment
- Know the timeline: establish gestational age and which structures/milestones should be present now.
- Interpret the tool: fundal height, ultrasound findings, heart tones, or movement pattern each measure a different aspect of growth or well-being.
- Connect to the system: a finding is a clue about the fetus (growth, maturity) or the support system (fluid, placenta, cord).
- Screen for red flags (decreased movement, abnormal fluid, growth concerns) and route them to the provider per policy.
- Teach the family what was checked, what it means, and what to report — in plain language, without overpromising certainty.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Gestational age | Fetal (conceptual) age | Counted from LMP vs. from conception — about a 2-week gap |
| Embryonic period | Fetal period | Organ formation (weeks 3–8) vs. growth/maturation (week 9–birth) |
| Umbilical vein | Umbilical arteries | Vein carries oxygenated blood to the fetus; arteries carry deoxygenated blood away — reverse of adult naming |
| Viability | Maturity | Able to survive with support vs. functionally ready (e.g., lungs) |
| "Small for dates" | "Something is wrong" | A surveillance-triggering description; many small babies are healthy |
| Quickening | Fetal heart tones | Felt movement (second trimester) vs. audible heartbeat (earlier, via Doppler) |
| Placenta as shield | Placenta as filter | It exchanges and lets many substances through — it does not block alcohol or most drugs |
| Preterm birth | Postterm birth | Birth before term vs. after term — different risk profiles, different care |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Building a baby is like building a house. First you pour the foundation and put up the frame — that's the first two months, when all the rooms (organs) are created. Then you spend the rest of the time furnishing, painting, and making everything work. If something harmful happens while the frame is going up, a room might be missing or misshapen. That's why the early weeks are the most delicate — and why, once the frame is built, the job becomes about growing bigger and getting ready to move in.
Worked example
Scenario: At a third-trimester visit, a pregnant person asks the nurse, "They said my baby is 'small for dates.' Does that mean something is wrong?" The nurse explains the distinction the visit revealed: gestational age by dates suggests one size expectation, but the fundal height trend and ultrasound measurements put this baby smaller than most peers at this age — the pattern is called "small for gestational age," a description that triggers closer surveillance, not a diagnosis of a problem. The nurse explains the team will monitor growth and fetal well-being more frequently, and that many small-for-gestational-age babies are simply constitutionally small. The nurse documents the teaching, reinforces the movement-counting instructions, and reviews the "call us if" list. The takeaway: fetal assessment is about patterns over time, and the nurse translates surveillance language into understandable, non-alarmist teaching without minimizing real risk.
Key takeaways
- Three periods: germinal (conception–2 wk), embryonic (3–8 wk, organogenesis, teratogen window), fetal (9 wk–birth, growth and maturation).
- Gestational age (from LMP) ≈ fetal age + 2 weeks — always know which clock is being used.
- The cord has 2 arteries and 1 vein — the reverse of adult circulation; oxygenated blood returns to the fetus through the vein.
- The placenta exchanges, it does not shield — alcohol and many drugs cross it. A safety cornerstone.
- Quickening is felt in the second trimester — earlier in subsequent pregnancies.
- Viability ≠ maturity: a fetus may be viable yet have immature lungs and brain; surfactant production drives lung readiness.
- Fetal shunts (ductus venosus, foramen ovale, ductus arteriosus) bypass the nonfunctional fetal lungs and close after birth.
- Head-down (cephalic) is the expected presentation at term; breech and other presentations affect birth planning.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name the three periods of prenatal development and the approximate weeks of each.
Show answer
Germinal (conception–~2 weeks), embryonic (~weeks 3–8), fetal (week 9–birth).
Why is the embryonic period the classic teratogen window?
Show answer
Because organogenesis — the formation of all major structures — happens in weeks 3–8, so harmful exposures during this window can produce structural defects; timing and dose determine the effect.
A newborn's cord has two arteries and one vein. Why is the vein the "oxygenated" vessel, and how does this differ from adult circulation?
Show answer
The umbilical vein returns oxygenated blood from the placenta to the fetus, and the arteries carry deoxygenated blood away — the reverse of the adult pattern, where arteries carry oxygenated blood.
What is the difference between gestational age and fetal age?
Show answer
Gestational age is counted from the last menstrual period; fetal (conceptual) age is counted from conception — about two weeks less.
Why is surfactant production the key to lung maturity?
Show answer
Surfactant lowers surface tension in the air sacs so they stay open after birth; without enough of it, the newborn cannot sustain breathing.
What is quickening, and why does its timing differ between first and later pregnancies?
Show answer
Quickening is the first felt fetal movement, typically in the second trimester — later for first pregnancies (the person doesn't yet recognize the sensation) and earlier in later pregnancies (familiarity and uterine muscle tone).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Germinal period
- First ~2 weeks: cell division and implantation
- Embryonic period
- Weeks 3–8: organ formation (organogenesis)
- Fetal period
- Week 9–birth: growth and functional maturation
- Gestational age
- Age counted from the last menstrual period
- Teratogen
- Any exposure that can harm fetal development
- Placenta
- The exchange organ between maternal and fetal circulations
- Wharton's jelly
- Protective cushioning around cord vessels
- Amniotic fluid
- Fluid surrounding and protecting the fetus
- Quickening
- First felt fetal movements
- Surfactant
- Substance that keeps lung air sacs open
- Fetal shunts
- Ductus venosus, foramen ovale, ductus arteriosus
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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