According to American College of Obstetricians and Gynecologists, the main methods to calculate<unk>  are: *Directly calculating the<unk>  since the beginning of the last menstrual period *Early obstetric ultrasound, comparing the size of an embryo or fetus to that of a reference group of pregnancies of known<unk>  (such as calculated from last menstrual periods) and using the mean<unk>  of other embryos or fetuses of the same size. If the<unk>  as calculated from an early ultrasound is contradictory to the one calculated directly from the last menstrual period, it is still the one from the early ultrasound that is used for the rest of the pregnancy. *In case of in vitro fertilization, calculating<unk>  since oocyte retrieval or co-incubation and adding 14<unk> .<unk>  can also be estimated by calculating<unk>  from ovulation if it was estimated from related signs or ovulation tests, and adding 14<unk>  by convention. A more complete listing of methods is given in following table: Method of estimating<unk>  Variability (2 standard deviations)<unk>  from oocyte retrieval or co-incubation in in vitro fertilisation + 14<unk>  ±1 day<unk>  from estimated ovulation in Ovulation induction + 14<unk>  ±3<unk> <unk>  from artificial insemination + 14<unk>  ±3<unk> <unk>  from known single sexual intercourse + 14<unk>  ±3<unk> <unk>  from estimated ovulation by basal body temperature record + 14<unk>  ±4<unk>  First-trimester physical examination ±2<unk>  Second-trimester physical examination ±4<unk>  Third-trimester physical examination ±6<unk>  First-trimester obstetric ultrasonography (crown-rump length) ±8% of the estimate Second-trimester obstetric ultrasonography (head circumference, femur length) ±8% of the estimate Third-trimester obstetric ultrasonography (head circumference, femur length) ±8% of the estimate As a general rule, the official<unk>  should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone. For example, if there is a<unk>  based on the beginning of the last menstrual period of 9.0<unk> , and a first-trimester obstetric ultrasonography gives an estimated<unk>  of 10.0<unk>  (with a 2 SD variability of ±8% of the estimate, thereby giving a variability of ±0.8<unk> , the difference of 1.0<unk>  between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the<unk>  estimated by ultrasonography should be used as the official<unk> . Once the estimated due date (EDD) is established, it should rarely be changed, as the determination of<unk>  is most accurate earlier in the pregnancy. Following are diagrams for estimating<unk>  from obstetric ultrasound, by various target parameters: File:Gestational sac diameter by<unk> .By<unk>  sac diameter File:Crown-rump length by<unk> .By crown-rump length (CRL) File:Biparietal diameter by<unk> .By biparietal diameter (BPD)
Gestational age
Metabolic age
Prenatal nutrition
Birth weight
Infant formula
Birthmark
Childbirth positions
Sex after pregnancy