2 Pregnancy Assessment and Fetal Development

Purpose of pregnancy assessment

Pregnancy assessment brings together accurate dating, observation of maternal adaptation, and repeated evaluation of fetal growth and well-being. Screening estimates risk or identifies conditions that may affect care; it does not replace clinical assessment or diagnostic testing.

Maternal adaptations

Pregnancy produces expected adaptations across multiple organ systems. These common patterns help distinguish typical changes from findings that need evaluation, but they do not remove the need to assess symptoms or abnormal measurements in context.

• Cardiovascular and blood: Blood volume and cardiac output increase. Plasma volume rises more than red-cell mass, which can lower measured hemoglobin through dilution. Blood pressure often falls somewhat in mid-pregnancy and may return toward baseline later.

• Respiratory: Tidal volume and minute ventilation increase, as does oxygen demand. Mild breathlessness with exertion can occur.

• Renal and urinary: Kidney filtration increases, and urinary frequency is common. The growing uterus and hormonal effects can dilate the urinary tract.

• Gastrointestinal: Slower smooth-muscle activity and pressure from the uterus can contribute to constipation and reflux.

• Endocrine and skin: Placental hormones help sustain pregnancy and increase insulin resistance. Increased pigmentation, including linea nigra or melasma, may occur.

Dating pregnancy

Gestational age is counted from the first day of the last menstrual period (LMP), approximately $2$ weeks before conception in a typical cycle. The estimated due date (EDD) is conventionally $280$ days, or $40$ weeks, after the LMP. Naegele’s rule estimates the date by adding $7$ days and subtracting $3$ months from the LMP.

This estimate assumes a reasonably regular $28$-day cycle. An uncertain LMP or irregular cycles can reduce its accuracy. A first-trimester ultrasound using crown–rump length is the most accurate ultrasound method for establishing or confirming gestational age.

Once the best estimated due date is established, it should generally remain the reference for care. Later scans are less accurate for changing dates because fetal size varies. For pregnancies conceived through assisted reproductive technology, use the dates and embryo age from treatment. A pregnancy without an ultrasound confirming or revising the EDD before $22$ weeks is considered suboptimally dated.

Pregnancy is commonly divided into three periods:

• The first trimester continues through $13$ weeks and $6$ days.

• The second trimester runs from $14$ weeks through $27$ weeks and $6$ days.

• The third trimester runs from $28$ weeks until birth.

Document gestational age in completed weeks and days. For example, $20$ weeks and $3$ days is written as $20\frac{3}{7}$ weeks.

Fetal development

Early development proceeds from implantation and rapid cell differentiation to formation of major body structures. The embryonic period is especially important for organ formation. By about $10$ weeks of gestational age, the developing baby is generally termed a fetus.

During the second trimester, the fetus grows, moves, and continues organ development. Fetal movement is often first perceived by the pregnant person around $16$–$20$ weeks, although timing varies. During the third trimester, the brain and lungs continue to mature, and the fetus gains substantial weight and fat.

These developmental milestones are approximate. They cannot determine an individual fetus’s health or exact maturity.

Assessing fetal growth

Growth is assessed over time rather than from a single measurement. During visits, clinicians may measure fundal height—the distance from the pubic bone to the top of the uterus—and compare the pattern with gestational age.

Fundal height is a screening measure. A size discrepancy or another concern may prompt ultrasound assessment of fetal size, fluid, anatomy, or placental and umbilical blood flow.

Prenatal screening and testing

Screening choices should be discussed with the patient, including each test’s purpose, limitations, and possible next steps. Patients may accept or decline screening and diagnostic testing.

Chromosomal screening

Cell-free DNA (cfDNA) screening can be offered from about $10$ weeks and is the most sensitive and specific screening option for the common trisomies $21$, $18$, and $13$. Serum-based approaches are alternatives. A positive or nonreportable result is not a diagnosis. Counseling, ultrasound assessment, and the option of diagnostic testing such as chorionic villus sampling (CVS) or amniocentesis should be discussed. Screening methods should not be combined indiscriminately as multiple independent screens.

Neural-tube-defect and anatomy assessment

Maternal serum alpha-fetoprotein (AFP) may be offered in the second trimester. An anatomic ultrasound is recommended for all pregnancies, ideally at $18$–$22$ weeks, regardless of the aneuploidy screening method.

Maternal laboratory screening

Early prenatal testing commonly includes a complete blood count, blood type and Rh status, urine testing and culture, and screening for relevant infections. Testing during each pregnancy for HIV, hepatitis B, hepatitis C, and syphilis is recommended. Repeat testing may be appropriate depending on risk and current guidance.

Later-pregnancy screening

Gestational-diabetes screening is usually performed at $24$–$28$ weeks. Group B streptococcus screening is generally performed late in pregnancy, with timing guided by current clinical recommendations.

A screening test estimates probability; a diagnostic test examines fetal or placental material to determine whether a particular condition is present. A positive screen does not confirm a condition, and a negative screen does not exclude every possible disorder.

Ongoing maternal and fetal assessment

Prenatal visits commonly include assessment of maternal history and symptoms, blood pressure, weight or weight-gain pattern, and indicated laboratory results. Fetal assessment may include fetal heart tones and, as pregnancy advances, fundal height and reported movement.

Blood pressure is particularly important to monitor for hypertensive disorders. Growth measurements help identify pregnancies that may need further evaluation. Visit frequency and additional tests are individualized according to health history, findings, and pregnancy risk.

Nonstress testing, biophysical profiles, and Doppler studies are additional, indication-based assessments rather than universal tests for every uncomplicated pregnancy. They may be used when a maternal or fetal condition raises concern about well-being or growth.

• Biophysical profile