Dating a pregnancy without a period
However, once a fetus is delivered, it is no longer at risk of intrauterine fetal demise, and use of this denominator has traditionally underestimated the risk of stillbirth.
The appropriate denominator is not all deliveries at a given gestational age, but ongoing (undelivered) pregnancies.
Ultrasonographic dating early in pregnancy can improve the reliability of the EDD; however, it is necessary to understand the margin of error reported at various times during each trimester.
A calculated gestational age by composite biometry from a sonogram must be considered an estimate and must take into account the range of possibilities. For example, crown-rump length (CRL) is 3-5 days, ultrasonography performed at 12-20 weeks of gestation is 7-10 days, at 20-30 weeks is 2 weeks, and after 30 weeks is 3 weeks.
While much of the work above has been conducted in postterm pregnancies.
Some of the fetal risks such as presence of meconium, increased risk of neonatal acidemia, and even stillbirth have been described as being greater at 41 weeks of gestation and even at 40 weeks of gestation as compared with 39 weeks’ gestation.
Traditionally, stillbirth rates were calculated using all pregnancies delivered at a given gestational age as the denominator.For example, in one study, the rates of meconium and neonatal acidemia both increased throughout term pregnancies beyond 38 weeks of gestation.In addition to stillbirth being increased prior to 42 weeks of gestation, one study found that the risk of neonatal mortality also increases beyond 41 weeks of gestation.If such a cycle is 35 days instead of 28 days, a second trimester ultrasound will not be powerful enough to redate the pregnancy.Thus, not only the LMP date, but the regularity and length of cycles must be taken into account when estimating gestational age.