Gestation is the period of fetal development from the time of conception to birth. Gestational age (or menstrual age), as defined by the American Academy of Pediatrics (AAP), is the “time elapsed between the first day of the last menstrual period and the day of delivery.” Gestational age is expressed in completed weeks (26-week and 4-day-old fetus is expressed as a 26-week fetus). Gestational age assessment is important for the obstetrician for obstetric care and management. Gestational age assessment is extremely important for the neonatologist for evaluation of the infant and to anticipate high-risk infants and complications. Gestational age and birthweight classification helps the neonatologist to categorize infants, guide treatment, and assess risks for morbidity and mortality. Neonates can be classified based on gestational age (preterm, late preterm, term, post term), birthweight (extremely low birthweight [ELBW], very low birthweight [VLBW], low birthweight [LBW], etc.), and gestational age and birthweight combined (small for gestational age [SGA], appropriate for gestational age [AGA], large for gestational age [LGA]). The AAP recommends that all newborns be classified by birthweight and gestational age.
I. GESTATIONAL AGE ASSESSMENT
Gestational age can be determined prenatally and postnatally.
Prenatal gestational age assessment. Determined by maternal history, clinical examination, and ultrasound examination. Based on these, the obstetrician is able to give his/or her “best estimate” of gestational age, since variability as much as 2 weeks can occur.
Date of last menstrual period. Reliable if dates remembered. The first day of the last menstrual period is about 2 weeks before ovulation and about 3 weeks before blastocyst implantation.
Assisted reproductive technology. In vitro fertilization pregnancies have a known date of conception and can accurately predict gestational age within 1 day. If pregnancy was achieved by using assisted reproductive technology, the gestational age is calculated by adding 2 weeks to the chronological age (time elapsed from birth). Intrauterine insemination may have a few days' delay.
Quickening. Date of first reported fetal activity by the mother (18–20 weeks for a primigravida, 15–17 weeks for a multipara).
Pelvic examination. Uterine size by bimanual examination in the first trimester can be accurate within 2 weeks.
Symphysis pubis fundal height. This is accurate up to 28–30 weeks' gestation. In resource-poor countries, gestational age can be estimated from serial measurements of symphysis pubis fundal height. It is only accurate within 4 weeks. One centimeter is equal to 1 week from the 18th to 20th weeks of gestation. At 20 weeks the fundus is at the umbilicus, and at term it is at the xiphoid process.
First fetal heart tones by Doppler ultrasound heard at 8–10 weeks.
Fetal heart motion/beat by ultrasound. Cardiac activity on ultrasound is detectable at 5.5–6.5 weeks by vaginal ultrasound, and 6.5–7 weeks by fetal ultrasound.
First trimester examination