title: Impact of ovulation and implantation timing on first-trimester crown-rump length and gestational age.
abstract: OBJECTIVE
  To determine the impact of ovulation and implantation timing on first-trimester crown-rump length (CRL) and the derived gestational age (GA).
  
  
  METHODS
  One hundred and forty-three women who were trying to conceive were recruited prospectively. The timing of ovulation and implantation and the ovulation to implantation (O-I) interval were established in 101 pregnancies using home urinary tests for luteinizing hormone and human chorionic gonadotropin. In 71 ongoing pregnancies, GA determined by measurement of fetal CRL at 10-14 weeks' gestation was compared with GA based on ovulation and implantation day. First-trimester growth was determined by serial ultrasound scans at 6-7, 8-9 and 10-14 weeks.
  
  
  RESULTS
  The median ovulation and implantation days were 16 and 27, respectively, with an O-I interval of 11 days. GA estimated from CRL at 10-14 weeks was on average 1.3 days greater than that derived from ovulation timing. CRL Z-score was inversely related to O-I interval (ρ= -0.431, P=0.0009). There was no significant relationship between CRL growth rate and the difference between observed CRL and expected CRL based on GA from last menstrual period (ρ=0.224, P=0.08).
  
  
  CONCLUSIONS
  Early implantation leads to a larger CRL and late implantation to a smaller CRL at 10-14 weeks, independent of CRL growth rate. Implantation timing is a major determinant of fetal size at 10-14 weeks and largely explains the variation in estimates of GA in the first trimester derived from embryonic or fetal CRL.
authors.affiliation: Fetal Medicine Department, Rosie Hospital, Addenbrooke's Hospital, Cambridge University Hospitals NHS Foundation Trust, Cambridge, UK.
authors.forename: A A
authors.initials: AA
authors.lastname: Mahendru
authors.forename: A
authors.initials: A
authors.lastname: Daemen
authors.forename: T R
authors.initials: TR
authors.lastname: Everett
authors.forename: I B
authors.initials: IB
authors.lastname: Wilkinson
authors.forename: C M
authors.initials: CM
authors.lastname: McEniery
authors.forename: Y
authors.initials: Y
authors.lastname: Abdallah
authors.forename: D
authors.initials: D
authors.lastname: Timmerman
authors.forename: T
authors.initials: T
authors.lastname: Bourne
authors.forename: C C
authors.initials: CC
authors.lastname: Lees
mesh_terms: D000328:Adult; D018568:Crown-Rump Length; D010064:Embryo Implantation / Q000502:physiology*; D005260:Female; D047109:Fetal Development / Q000502:physiology*; D005865:Gestational Age; D006801:Humans; D010060:Ovulation / Q000502:physiology*; D011247:Pregnancy; D011261:Pregnancy Trimester, First; D011446:Prospective Studies; D016216:Ultrasonography, Prenatal