Heterogeneity in fetal growth velocity
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, smokingand folic acid intake. 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. Some authorities however casually interchange these terms[ citation needed ] and the reader is advised to be cautious.
On average, this is 9 months and 6 days.
should be used to assess fetal size and should not normally be used to reassign gestational age. For dating charts the known variable [crown-rump length. PDF, On Aug 1,, Pam Loughna and others published Fetal Size and Dating: Charts Recommended for Clinical Obstetric Practice, Find.