Embryonic and fetal development

Gestational age and time since fertilization

Clinical gestational age is usually counted from the first day of the last menstrual period rather than fertilization. In a typical example it is about two weeks greater than postfertilization age, but variable ovulation changes that relationship. Early ultrasound and history help establish an appropriate estimate. A textbook’s eighth week after fertilization therefore differs from a report’s eighth gestational week. Always identify the convention before interpreting developmental timing. After fertilization, the zygote divides, implantation occurs, and cells acquire different functions as structures emerge. The embryonic period generally covers the first eight postfertilization weeks, followed by the fetal period, emphasizing growth and further maturation—approximately after ten gestational weeks. This does not mean no growth occurred earlier or no differentiation continues later. More cells and a fully functional organ are different achievements.

Sources: MedlinePlus: Fetal development.

The placenta is a developing exchange organ

Placental development involves embryo-associated cells establishing connections with the uterine lining, with villous structures and maternal vessels changing to support exchange. Remodelling of uterine spiral arteries helps adapt blood supply. The placenta is not a finished filter present immediately after conception; it is a developing organ with transport, endocrine, and immune-related functions. Maternal and fetal circulations interact without becoming one freely mixed pool. Oxygen, nutrients, and metabolic products cross an exchange interface whose performance depends on blood flow, surface area, barrier properties, and transport mechanisms. Selectivity does not block every medicine, alcohol exposure, or pathogen. Abnormal supply or transport can affect growth, explaining why eating more cannot solve every case of a small fetus. Interpretation must separate nutrition, placental performance, fetal factors, and gestational-age estimation rather than relying on weight alone.

Sources: NICHD: Placental development.

Placental oxygenation requires a different circulation

The fetal lungs do not yet perform postnatal gas exchange; oxygen comes primarily through the placenta. The umbilical vein carries relatively oxygen-rich blood toward the fetus, while umbilical arteries return blood to the placenta. Vessels are named by direction of flow, not an absolute rule that arteries carry more oxygen. Some umbilical venous blood passes through the ductus venosus, bypassing part of the hepatic circulation. The foramen ovale and ductus arteriosus allow substantial blood flow to bypass the lungs. These pathways have normal functions before birth. Lung expansion and changing resistance and pressure relationships subsequently shift circulation toward the newborn pattern. A structure can therefore mean different things at different ages; an opening or duct should not automatically be interpreted through adult disease concepts. This also helps explain changing manifestations of some congenital heart conditions after birth.

Sources: American Heart Association: Fetal circulation.

Formation, growth, and maturation differ

Early development establishes basic organ form, followed by continued proliferation, connections, and functional maturation. Increasing size later in pregnancy does not make the brain, lungs, or other systems identical to those of a full-term newborn. Movement also does not prove completion of every neurological function. Development depends on gestational age, genetics, placenta, and maternal conditions, with normal variation; fruit-size analogies cannot assess health. Prenatal observation considers growth over time relative to gestational age and other evidence. A small measurement may reflect dating, measurement error, constitutional differences, or growth restriction, requiring different interpretations. Prematurity is likewise more than low weight: two equally heavy newborns may differ in organ maturity because they were born at different gestational ages. Decisions about earlier delivery balance continuing-pregnancy risks against postnatal risks instead of mechanically pursuing a target weight.

Sources: MedlinePlus: Fetal development; NICHD: Pregnancy.

Exposure risk depends on substance, dose, and timing

Developing tissues differ in susceptibility across stages, and risk also depends on substance, dose, duration, and individual conditions. Having encountered something during pregnancy is therefore an incomplete exposure description: it neither guarantees safety nor establishes injury. There is no known safe amount or safe period for alcohol use in pregnancy. Stopping and obtaining support is useful, but this preventive advice does not predict an individual fetus’s outcome from one previous exposure. Medicine decisions must weigh treated and untreated disease, avoiding abrupt self-discontinuation of long-term prescriptions. Infection prevention, food safety, vaccination, and occupational exposure assessment also require context. Lin worries about a cold medicine taken before recognizing pregnancy. Recording the product, dose, and timing for professional review is more useful than searching only for a blanket pregnancy prohibition. Specific information enables an evidence-based assessment rather than diffuse fear.

Sources: CDC: Alcohol during pregnancy; NICHD: Prenatal care; CDC: During pregnancy.

Prenatal care integrates several kinds of evidence

Prenatal care considers maternal blood pressure, illness, nutrition, mood, and circumstances alongside fetal health. Ultrasound provides selected structural and growth information; screening estimates selected risks; diagnostic procedures answer more specific questions. Noninvasive prenatal screening analyzes cell-free DNA in maternal blood, including placental contributions. It is screening, not definitive testing for every genetic disorder. Increased-risk results usually require counselling and diagnostic confirmation, while reduced-risk results cannot exclude every structural or functional problem. If Lin has a slightly small ultrasound measurement and low-risk screening, the latter does not invalidate the former or combine into one overall passing score. Ask what each test measured, consider dating and trends, and follow the clinical plan. Severe persistent headache, visual changes, chest pain or breathing difficulty, significant bleeding, or markedly reduced established fetal movement require immediate contact with the clinical team or emergency services rather than waiting for a routine visit.

Sources: NICHD: Prenatal care; MedlinePlus Genetics: NIPT; CDC: Urgent maternal warning signs.

Bilingual terms

孕周 · Gestational age

Pregnancy age usually dated from the first day of the last menstrual period.

着床 · Implantation

Establishment of the early embryo within the uterine lining.

胎盘 · Placenta

A temporary organ supporting exchange and endocrine functions.

卵圆孔 · Foramen ovale

A normal fetal connection between the atria.

产前筛查 · Prenatal screening

Testing that estimates risk of selected fetal conditions.

Sources and further reading

• MedlinePlus: Fetal development
• NICHD: Placental development
• American Heart Association: Fetal circulation
• NICHD: Pregnancy
• CDC: Alcohol during pregnancy
• NICHD: Prenatal care
• MedlinePlus Genetics: NIPT
• CDC: Urgent maternal warning signs
• CDC: During pregnancy

Original course source-check record: 9 September 2026. Full Chinese and English sentence-by-sentence language review: 14 September 2026. AI editing and language review are not human clinical review. Linked institutions have not participated in or endorsed this course.