Ultrasound of human fetus, 8 weeks and 1 day

Crown-rump length (CRL) is the measurement of the length of human

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s and

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal dev ...

es from the top of the head (crown) to the bottom of the buttocks (rump). It is typically determined from ultrasound imagery and can be used to estimate gestational age.

Introduction

The embryo and fetus float in the

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between ...

inside the

uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...

of the mother usually in a curved posture resembling the letter ''C''. The measurement can actually vary slightly if the fetus is temporarily stretching (straightening) its body. The measurement needs to be in the natural state with an unstretched body which is actually ''C'' shaped. The measurement of CRL is useful in determining the gestational age (menstrual age starting from the first day of the last menstrual period) and thus the expected date of delivery (EDD). Different babies do grow at different rates and thus the gestational age is an approximation. Recent evidence has indicated that CRL growth (and thus the approximation of gestational age) may be influenced by maternal factors such as age,

smoking Smoking is a practice in which a substance is burned and the resulting smoke is typically breathed in to be tasted and absorbed into the bloodstream. Most commonly, the substance used is the dried leaves of the tobacco plant, which have b ...

, and folic acid intake. Early in

pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...

gestational age 8 weeks, it is accurate within about +/- 5 days but later in pregnancy due to different growth rates, the accuracy is less. In that situation, other parameters can be used in addition to CRL. The length of the

umbilical cord In placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord is physiologi ...

is approximately equal to the CRL throughout pregnancy. Gestational age is not the same as fertilization age. It takes about 14 days from the first day of the last menstrual period for conception to take place and thus for the conceptus to form. The age from this point in time (conception) is called the fertilization age and is thus 2 weeks shorter than the gestational age. Thus a 6-week gestational age would be a 4-week fertilization age. Some authorities however casually interchange these terms and the reader is advised to be cautious. An average gestational period (duration of pregnancy from the first day of the last menstrual period up to delivery) is 280 days. On average, this is 9 months and 6 days.

Gestational age estimation

A commonly used estimate of gestational age in weeks is (as described by Verburg et al.):

GA_=CRL^\cdot e^

Gestational age estimation in days is carried out according to the equations:

GA_=40.9+3.24585\cdot CRL^+0.348956\cdot CRL

; and SD of GA = 2.39102 + (0.0193474 × CRL).

References

{{Reflist Embryology Obstetrics Midwifery

Introduction

Gestational age estimation

See also

References