Gestational Age, Forensic Determination

Estimating gestational age when a fetus dies is a specialized task in forensic medicine . Techniques for determining the gestational age of fetal or perinatal (around the time of birth) remains are mainly aimed at calculating the time since conception, and at determining if a specific disease could be the cause of fetal loss. In some cases, the time of conception cannot be known with certainty, as calculations based on last normal menstrual period may lead to an error of as much as two weeks.

The World Health Organization has set the viability threshold at 20 weeks gestation. Fetal death is defined as the death of a product of conception (fetus) before complete expulsion or extraction from its mother, regardless of the duration of pregnancy. Fetal death is divided into three categories: early, intermediate, and late. Early fetal death occurs at less than 20 completed weeks of gestation; intermediate fetal death occurs from week 20 through week 27 of gestation; and late fetal death occurs at 28 or more completed weeks of gestation. Proper classification of the fetal age into the correct classification has important ethical, legal, and also clinical implications.

Depending on the general condition of fetal remains, forensic specialists might face difficulties with age estimation. The whole skeletal length was probably the first marker used for fetal assessment and is considered valuable for diagnosing various syndromes and skeletal dysplasias (abnormal development) as well as for assessing fetal development. However, even if still considered a marker of developmental age, the whole skeletal length may be affected in post mortem (after death) assessment by the putrefactive (decomposition ) process. In particular, body length increases slightly with maceration (tissue softening in liquid), whereas body weight and head circumference seem to be unaffected. For this reason, long bone length is considered a more stable and reliable marker than full-body measurements. Several investigators have produced linear regression formulas based on crown-heel length, crown-rump length, or body diameters to determine gestational age.

The advent of ultrasonography about three decades ago, especially high-resolution real-time ultrasonography, allowed a more accurate determination of fetal gestational age (crown-rump length or biparietal diameter and/or femur length). This technology is useful in forensic measurement of bones from standardized post-mortem radiographs in cases of questionable gestational age and can be compared with previous ultrasonographic measurements. Reference tables correlated with in vivo (in life) ultrasound measurements are widely available for complete fetuses and fetal remains including soft tissues, but the forensic specialist would also need methods for estimating gestational age estimation for osseous (skeletal, bone) remains.

One relatively accurate radiographic (x ray or other radiographic image) protocol for estimating fetal gestational age at death is based on femur (the long bone in the leg) diaphyseal (shaft) length, and compares images of fetal femur measurements with measurements of the same bones at autopsy . As several organs show major changes in developmental patterns throughout fetal development, maturation of fetal tissues and organs has been also proposed for gestational age estimation. One study evaluated the histological maturation of several soft tissues (skin, thymus, lungs, thyroid gland, kidneys, adrenal glands, and central nervous system) from 448 normal fetuses between 12–40 weeks gestation, and compared the estimated gestational age with that obtained by long bone measurement. Skin, lung, and kidney tissue (each with unique but distinguishable stages of development) were found to be useful for a more accurate assessment of gestational age when integrated with long bones measurements. In another study, the adrenal glands were found to be useful to define fetal age and maturation. Finally, another study reported that fetal development is constant and is not affected by intrauterine malnutrition and chromosomal abnormalities. Patterns of fetal skin development vary by site, with a range of 1–2 weeks in different regions of the body, and can sometimes give a rough suggestion of gestational age.

The growth of fetal long bones is affected by several conditions that might lead to growth retardation of the fetus in the womb. For this reason, many studies about estimating post-mortem gestational age have addressed the need for identifying new technologies and new targets to measure, aside from the long bones.

In conclusion, macroscopic examination (with the eye)or radiographic (x ray) examination are the most common methods used to estimate fetal age. Examination of the tissues and organs is also important for a better definition of fetal age.

see also Autopsy; Coffin birth; Skeletal analysis.