Smarter Decisions,
Better Care

UpToDate synthesizes the most recent medical information into evidence-based practical recommendations clinicians trust to make the right point-of-care decisions.

  • Rigorous editorial process: Evidence-based treatment recommendations
  • World-Renowned physician authors: over 5,100 physician authors and editors around the globe
  • Innovative technology: integrates into the workflow; access from EMRs

Choose from the list below to learn more about subscriptions for a:


Subscribers log in here


Prenatal assessment of gestational age and estimated date of delivery

INTRODUCTION

Ultrasonography has advanced obstetric practice by enabling relatively detailed assessment of the fetus in utero, including an accurate estimate of gestational age when performed in the first half of pregnancy [1]. This information is invaluable because most obstetrical management decisions are strongly influenced by consideration of fetal development, which closely correlates with fetal age. A first trimester ultrasound examination can decrease the rate of induction for postterm pregnancy [2] and improve estimation of viability when complications occur at the limit of viability. (See "Limit of viability".)

Fetal biometric measurements used to calculate gestational age and estimated date of delivery ("due date" or EDD) will be reviewed here. The discussion of pregnancy dating will be based on time since the last menstrual period (LMP, menstrual age), not time since conception (conceptional or embryonic age).

ESTIMATED DATE OF DELIVERY

The estimated date of delivery (EDD) is 280 days from the onset of the last menstrual period (LMP) and 266 days from date of conception. Although fertilization of an oocyte in vivo cannot be detected by any laboratory or imaging method, it occurs within 24 hours of ovulation, which can be accurately predicted by detecting the luteinizing hormone (LH) surge or by ultrasound examination of the periovulatory follicle (see "Evaluation of the menstrual cycle and timing of ovulation") Only 4 percent of women deliver on their EDD, in part because of the limitations of methods used to estimate gestational age, but also because of natural biologic variation in the pace of fetal maturation and the timing of natural delivery [3].

CLINICAL ASSESSMENT

The two clinical methods of gestational age assessment are history, using the date of the last menstrual period (LMP) to calculate the estimated date of delivery (EDD), and physical examination. The clinical assessment of gestational age or duration of pregnancy reflects the "menstrual age." In contrast, embryologists always date developmental events from the time of fertilization, which is the "embryonic age."

Naegele's rule — Naegele's rule is the most common method of pregnancy dating. The EDD is calculated by counting back three months from the LMP and adding seven days. As an example, if the LMP is February 20, then the EDD will be November 27. If the LMP is May 28, then the EDD will be March 4. This method assumes the patient has a 28-day menstrual cycle with fertilization occurring on day 14.

                     

Subscribers log in here

To continue reading this article, you must log in with your personal, hospital, or group practice subscription. For more information or to purchase a personal subscription, click below on the option that best describes you:
Literature review current through: Sep 2014. | This topic last updated: Sep 30, 2014.
The content on the UpToDate website is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding any medical questions or conditions. The use of this website is governed by the UpToDate Terms of Use ©2014 UpToDate, Inc.
References
Top
  1. Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol 1996; 174:278.
  2. Bennett KA, Crane JM, O'shea P, et al. First trimester ultrasound screening is effective in reducing postterm labor induction rates: a randomized controlled trial. Am J Obstet Gynecol 2004; 190:1077.
  3. Jukic AM, Baird DD, Weinberg CR, et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod 2013; 28:2848.
  4. Creinin MD, Keverline S, Meyn LA. How regular is regular? An analysis of menstrual cycle regularity. Contraception 2004; 70:289.
  5. Geirsson RT, Busby-Earle RM. Certain dates may not provide a reliable estimate of gestational age. Br J Obstet Gynaecol 1991; 98:108.
  6. Johnson SR, Miro F, Barrett S, Ellis JE. Levels of urinary human chorionic gonadotrophin (hCG) following conception and variability of menstrual cycle length in a cohort of women attempting to conceive. Curr Med Res Opin 2009; 25:741.
  7. Chambliss LR, Clark SL. Paper gestational age wheels are generally inaccurate. Am J Obstet Gynecol 2014; 210:145.e1.
  8. Neilson JP. Ultrasound for fetal assessment in early pregnancy. Cochrane Database Syst Rev 2000; :CD000182.
  9. Yang H, Kramer MS, Platt RW, et al. How does early ultrasound scan estimation of gestational age lead to higher rates of preterm birth? Am J Obstet Gynecol 2002; 186:433.
  10. Laing FC, Frates MC. Ultrasound Evaluation During the First Trimester of Pregnancy. In: Ultrasonography in Obstetrics and Gynecology, 4th ed, Callen PW (Ed), WB Saunders Co, Philadelphia 2000.
  11. Rowling SE, Langer JE, Coleman BG, et al. Sonography during early pregnancy: dependence of threshold and discriminatory values on transvaginal transducer frequency. AJR Am J Roentgenol 1999; 172:983.
  12. Grisolia G, Milano K, Pilu G, et al. Biometry of early pregnancy with transvaginal sonography. Ultrasound Obstet Gynecol 1993; 3:403.
  13. Fleischer AC, Kalemeris GC, Machin JE, et al. Sonographic depiction of normal and abnormal endometrium with histopathologic correlation. J Ultrasound Med 1986; 5:445.
  14. Forrest TS, Elyaderani MK, Muilenburg MI, et al. Cyclic endometrial changes: US assessment with histologic correlation. Radiology 1988; 167:233.
  15. Nyberg DA, Laing FC, Filly RA. Threatened abortion: sonographic distinction of normal and abnormal gestation sacs. Radiology 1986; 158:397.
  16. Yeh HC, Goodman JD, Carr L, Rabinowitz JG. Intradecidual sign: a US criterion of early intrauterine pregnancy. Radiology 1986; 161:463.
  17. Bradley WG, Fiske CE, Filly RA. The double sac sign of early intrauterine pregnancy: use in exclusion of ectopic pregnancy. Radiology 1982; 143:223.
  18. Doubilet PM, Benson CB. Double sac sign and intradecidual sign in early pregnancy: interobserver reliability and frequency of occurrence. J Ultrasound Med 2013; 32:1207.
  19. Timor-Tritsch IE, Farine D, Rosen MG. A close look at early embryonic development with the high-frequency transvaginal transducer. Am J Obstet Gynecol 1988; 159:676.
  20. Rossavik IK, Torjusen GO, Gibbons WE. Conceptual age and ultrasound measurements of gestational sac and crown-rump length in in vitro fertilization pregnancies. Fertil Steril 1988; 49:1012.
  21. de Crespigny LC, Cooper D, McKenna M. Early detection of intrauterine pregnancy with ultrasound. J Ultrasound Med 1988; 7:7.
  22. Bree RL, Edwards M, Böhm-Vélez M, et al. Transvaginal sonography in the evaluation of normal early pregnancy: correlation with HCG level. AJR Am J Roentgenol 1989; 153:75.
  23. Wong HS, Cheung YK, Tait J. Sonographic study of the decidua basalis in the first trimester of pregnancy. Ultrasound Obstet Gynecol 2009; 33:634.
  24. Nyberg DA, Mack LA, Laing FC, Patten RM. Distinguishing normal from abnormal gestational sac growth in early pregnancy. J Ultrasound Med 1987; 6:23.
  25. Robinson HP. "Gestation sac" volumes as determined by sonar in the first trimester of pregnancy. Br J Obstet Gynaecol 1975; 82:100.
  26. Daya S, Woods S, Ward S, et al. Early pregnancy assessment with transvaginal ultrasound scanning. CMAJ 1991; 144:441.
  27. Hellman LM, Kobayashi M, Fillisti L, et al. Growth and development of the human fetus prior to the twentieth week of gestation. Am J Obstet Gynecol 1969; 103:789.
  28. Nyberg DA, Filly RA, Mahony BS, et al. Early gestation: correlation of HCG levels and sonographic identification. AJR Am J Roentgenol 1985; 144:951.
  29. Filly RA, Hadlock FP. Sonographic Determination of Menstrual Age. In: Ultrasonography in Obstetrics and Gynecology, 4th ed, Callen PW (Ed), WB Saunders Co, Philadelphia 2000.
  30. Levi CS, Lyons EA, Lindsay DJ. Early diagnosis of nonviable pregnancy with endovaginal US. Radiology 1988; 167:383.
  31. Benson CB, Doubilet PM. Fetal measurements-normal and abnormal fetal growth. In: Diagnostic Ultrasound, 2nd ed, Rumack CM, Wilson SR, Charboneau JW (Eds), CV Mosby, St. Louis 1998. Vol 2, p.1013.
  32. Jauniaux E, Jurkovic D, Henriet Y, et al. Development of the secondary human yolk sac: correlation of sonographic and anatomical features. Hum Reprod 1991; 6:1160.
  33. Papaioannou GI, Syngelaki A, Poon LC, et al. Normal ranges of embryonic length, embryonic heart rate, gestational sac diameter and yolk sac diameter at 6-10 weeks. Fetal Diagn Ther 2010; 28:207.
  34. Lindsay DJ, Lovett IS, Lyons EA, et al. Yolk sac diameter and shape at endovaginal US: predictors of pregnancy outcome in the first trimester. Radiology 1992; 183:115.
  35. Stampone C, Nicotra M, Muttinelli C, Cosmi EV. Transvaginal sonography of the yolk sac in normal and abnormal pregnancy. J Clin Ultrasound 1996; 24:3.
  36. Nyberg DA, Hill LM. Normal early intrauterine pregnancy: Sonographic development and hCG correlation. In: Transvaginal Ultrasound, Nyberg DA, Hill LM, Bohm-Velez M, Mendelson EB (Eds), Mosby-Year Book, St. Louis 1992. p.65.
  37. Varelas FK, Prapas NM, Liang RI, et al. Yolk sac size and embryonic heart rate as prognostic factors of first trimester pregnancy outcome. Eur J Obstet Gynecol Reprod Biol 2008; 138:10.
  38. Reece EA, Scioscia AL, Pinter E, et al. Prognostic significance of the human yolk sac assessed by ultrasonography. Am J Obstet Gynecol 1988; 159:1191.
  39. Tan S, İpek A, Pektas MK, et al. Irregular yolk sac shape: is it really associated with an increased risk of spontaneous abortion? J Ultrasound Med 2011; 30:31.
  40. Cadkin AV, McAlpin J. Detection of fetal cardiac activity between 41 and 43 days of gestation. J Ultrasound Med 1984; 3:499.
  41. Nyberg DA, Mack LA, Harvey D, Wang K. Value of the yolk sac in evaluating early pregnancies. J Ultrasound Med 1988; 7:129.
  42. Goldstein SR, Wolfson R. Endovaginal ultrasonographic measurement of early embryonic size as a means of assessing gestational age. J Ultrasound Med 1994; 13:27.
  43. Yeh HC, Rabinowitz JG. Endovaginal ultrasonographic measurement of early embryonic size. J Ultrasound Med 1995; 14:97.
  44. Daya S. Accuracy of gestational age estimation by means of fetal crown-rump length measurement. Am J Obstet Gynecol 1993; 168:903.
  45. Wisser J, Dirschedl P, Krone S. Estimation of gestational age by transvaginal sonographic measurement of greatest embryonic length in dated human embryos. Ultrasound Obstet Gynecol 1994; 4:457.
  46. Robinson HP, Fleming JE. A critical evaluation of sonar "crown-rump length" measurements. Br J Obstet Gynaecol 1975; 82:702.
  47. Deter RL, Buster JE, Casson PR, Carson SA. Individual growth patterns in the first trimester: evidence for difference in embryonic and fetal growth rates. Ultrasound Obstet Gynecol 1999; 13:90.
  48. Robinson HP. Sonar measurement of fetal crown-rump length as means of assessing maturity in first trimester of pregnancy. Br Med J 1973; 4:28.
  49. Hadlock FP, Shah YP, Kanon DJ, Lindsey JV. Fetal crown-rump length: reevaluation of relation to menstrual age (5-18 weeks) with high-resolution real-time US. Radiology 1992; 182:501.
  50. Silva PD, Mahairas G, Schaper AM, Schauberger CW. Early crown-rump length. A good predictor of gestational age. J Reprod Med 1990; 35:641.
  51. Napolitano R, Dhami J, Ohuma EO, et al. Pregnancy dating by fetal crown-rump length: a systematic review of charts. BJOG 2014; 121:556.
  52. McLennan AC, Schluter PJ. Construction of modern Australian first trimester ultrasound dating and growth charts. J Med Imaging Radiat Oncol 2008; 52:471.
  53. Sahota DS, Leung TY, Leung TN, et al. Fetal crown-rump length and estimation of gestational age in an ethnic Chinese population. Ultrasound Obstet Gynecol 2009; 33:157.
  54. Verburg BO, Steegers EA, De Ridder M, et al. New charts for ultrasound dating of pregnancy and assessment of fetal growth: longitudinal data from a population-based cohort study. Ultrasound Obstet Gynecol 2008; 31:388.
  55. Drumm JE, Clinch J, MacKenzie G. The ultrasonic measurement of fetal crown-rump length as a method of assessing gestational age. Br J Obstet Gynaecol 1976; 83:417.
  56. MacGregor SN, Tamura RK, Sabbagha RE, et al. Underestimation of gestational age by conventional crown-rump length dating curves. Obstet Gynecol 1987; 70:344.
  57. Jeanty P. fetal Biometry. In: Sonography in Obstetrics and Gynecology – Principles and Practice, 6th ed, Fleischer AC, Manning FA, Jeanty P, Romero R (Eds), McGraw-Hill, 2001.
  58. Campbell S, Warsof SL, Little D, Cooper DJ. Routine ultrasound screening for the prediction of gestational age. Obstet Gynecol 1985; 65:613.
  59. Waldenström U, Axelsson O, Nilsson S. A comparison of the ability of a sonographically measured biparietal diameter and the last menstrual period to predict the spontaneous onset of labor. Obstet Gynecol 1990; 76:336.
  60. Hackmon R, Le Scale KB, Horani J, et al. Is severe macrosomia manifested at 11-14 weeks of gestation? Ultrasound Obstet Gynecol 2008; 32:740.
  61. Hadlock FP. Sonographic estimation of fetal age and weight. Radiol Clin North Am 1990; 28:39.
  62. Reece EA, Scioscia AL, Green J, et al. Embryonic trunk circumference: a new biometric parameter for estimation of gestational age. Am J Obstet Gynecol 1987; 156:713.
  63. Selbing A. Gestational age and ultrasonic measurement of gestational sac, crown-rump length and biparietal diameter during first 15 weeks of pregnancy. Acta Obstet Gynecol Scand 1982; 61:233.
  64. Bovicelli L, Orsini LF, Rizzo N, et al. Estimation of gestational age during the first trimester by real-time measurement of fetal crown-rump length and biparietal diameter. J Clin Ultrasound 1981; 9:71.
  65. Butt K, Lim K, Society of Obstetricians and Gynaecologists of Canada. Determination of gestational age by ultrasound. J Obstet Gynaecol Can 2014; 36:171.
  66. National Institute for Health and Clinical Excellence. Antenatal Care (CG62). http://guidance.nice.org.uk/CG62 (Accessed on April 22, 2014).
  67. Salomon LJ, Alfirevic Z, Bilardo CM, et al. ISUOG practice guidelines: performance of first-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol 2013; 41:102.
  68. Committee opinion no 611: method for estimating due date. Obstet Gynecol 2014; 124:863.
  69. Hadlock FP, Harrist RB, Martinez-Poyer J. How accurate is second trimester fetal dating? J Ultrasound Med 1991; 10:557.
  70. Rossavik IK, Fishburne JI. Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period. Obstet Gynecol 1989; 73:243.
  71. de Crespigny LC, Speirs AL. A new look at biparietal diameter. Aust N Z J Obstet Gynaecol 1989; 29:26.
  72. Hadlock FP, Deter RL, Harrist RB, Park SK. Fetal biparietal diameter: rational choice of plane of section for sonographic measurement. AJR Am J Roentgenol 1982; 138:871.
  73. Kurtz AB, Wapner RJ, Kurtz RJ, et al. Analysis of biparietal diameter as an accurate indicator of gestational age. J Clin Ultrasound 1980; 8:319.
  74. Kopta MM, May RR, Crane JP. A comparison of the reliability of the estimated date of confinement predicted by crown-rump length and biparietal diameter. Am J Obstet Gynecol 1983; 145:562.
  75. Persson PH, Weldner BM. Reliability of ultrasound fetometry in estimating gestational age in the second trimester. Acta Obstet Gynecol Scand 1986; 65:481.
  76. Kurtz AB, Needleman L. Ultrasound Assessment of Fetal Age, 2nd ed, WB Saunders Co, Philadelphia 1988.
  77. Selbing A, Kjessler B. Conceptual dating by ultrasonic measurement of the fetal biparietal diameter in early pregnancy. Acta Obstet Gynecol Scand 1985; 64:593.
  78. Benson CB, Doubilet PM. Sonographic prediction of gestational age: accuracy of second- and third-trimester fetal measurements. AJR Am J Roentgenol 1991; 157:1275.
  79. Shepard M, Filly RA. A standardized plane for biparietal diameter measurement. J Ultrasound Med 1982; 1:145.
  80. Hadlock FP, Deter RL, Harrist RB, Park SK. Fetal biparietal diameter: a critical re-evaluation of the relation to menstrual age by means of real-time ultrasound. J Ultrasound Med 1982; 1:97.
  81. Hadlock FP, Deter RL, Carpenter RJ, Park SK. Estimating fetal age: effect of head shape on BPD. AJR Am J Roentgenol 1981; 137:83.
  82. Wolfson RN, Zador IE, Halvorsen P, et al. Biparietal diameter in premature rupture of membranes: errors in estimating gestational age. J Clin Ultrasound 1983; 11:371.
  83. O'Keeffe DF, Garite TJ, Elliott JP, Burns PE. The accuracy of estimated gestational age based on ultrasound measurement of biparietal diameter in preterm premature rupture of the membranes. Am J Obstet Gynecol 1985; 151:309.
  84. Gray DL, Songster GS, Parvin CA, Crane JP. Cephalic index: a gestational age-dependent biometric parameter. Obstet Gynecol 1989; 74:600.
  85. Law RG, MacRae KD. Head circumference as an index of fetal age. J Ultrasound Med 1982; 1:281.
  86. Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology 1984; 152:497.
  87. Hadlock FP, Harrist RB, Shah YP, et al. Estimating fetal age using multiple parameters: a prospective evaluation in a racially mixed population. Am J Obstet Gynecol 1987; 156:955.
  88. Hill LM, Guzick D, Hixson J, et al. Composite assessment of gestational age: a comparison of institutionally derived and published regression equations. Am J Obstet Gynecol 1992; 166:551.
  89. Ott WJ. The use of ultrasonic fetal head circumference for predicting expected date of confinement. J Clin Ultrasound 1984; 12:411.
  90. Jeanty P, Rodesch F, Delbeke D, Dumont JE. Estimation of gestational age from measurements of fetal long bones. J Ultrasound Med 1984; 3:75.
  91. Ott WJ. Accurate gestational dating. Obstet Gynecol 1985; 66:311.
  92. Hadlock FP, Harrist RB, Deter RL, Park SK. Fetal femur length as a predictor of menstrual age: sonographically measured. AJR Am J Roentgenol 1982; 138:875.
  93. Hohler CW, Quetel TA. Fetal femur length: equations for computer calculation of gestational age from ultrasound measurements. Am J Obstet Gynecol 1982; 143:479.
  94. Goldstein RB, Filly RA, Simpson G. Pitfalls in femur length measurements. J Ultrasound Med 1987; 6:203.
  95. Mahony BS, Filly RA. High-resolution sonographic assessment of the fetal extremities. J Ultrasound Med 1984; 3:489.
  96. Hadlock FP, Harrist RB, Deter RL, Park SK. A prospective evaluation of fetal femur length as a predictor of gestational age. J Ultrasound Med 1983; 2:111.
  97. Papageorghiou AT, Fratelli N, Leslie K, et al. Outcome of fetuses with antenatally diagnosed short femur. Ultrasound Obstet Gynecol 2008; 31:507.
  98. Weisz B, David AL, Chitty L, et al. Association of isolated short femur in the mid-trimester fetus with perinatal outcome. Ultrasound Obstet Gynecol 2008; 31:512.
  99. Hadlock FP, Deter RL, Harrist RB, Park SK. Fetal abdominal circumference as a predictor of menstrual age. AJR Am J Roentgenol 1982; 139:367.
  100. Chinn DH, Filly RA, Callen PW. Ultrasonic evaluation of fetal umbilical and hepatic vascular anatomy. Radiology 1982; 144:153.
  101. Jeanty P, Cantraine F, Cousaert E, et al. The binocular distance: a new way to estimate fetal age. J Ultrasound Med 1984; 3:241.
  102. Hill LM, Guzick D, Fries J, et al. The transverse cerebellar diameter in estimating gestational age in the large for gestational age fetus. Obstet Gynecol 1990; 75:981.
  103. Reece EA, Goldstein I, Pilu G, Hobbins JC. Fetal cerebellar growth unaffected by intrauterine growth retardation: a new parameter for prenatal diagnosis. Am J Obstet Gynecol 1987; 157:632.
  104. Chavez MR, Ananth CV, Smulian JC, et al. Fetal transcerebellar diameter measurement with particular emphasis in the third trimester: a reliable predictor of gestational age. Am J Obstet Gynecol 2004; 191:979.
  105. Yarkoni S, Schmidt W, Jeanty P, et al. Clavicular measurement: a new biometric parameter for fetal evaluation. J Ultrasound Med 1985; 4:467.
  106. Mercer BM, Sklar S, Shariatmadar A, et al. Fetal foot length as a predictor of gestational age. Am J Obstet Gynecol 1987; 156:350.
  107. Platt LD, Medearis AL, DeVore GR, et al. Fetal foot length: relationship to menstrual age and fetal measurements in the second trimester. Obstet Gynecol 1988; 71:526.
  108. Hill LM, Guzick D, Thomas ML, Fries JK. Fetal radius length: a critical evaluation of race as a factor in gestational age assessment. Am J Obstet Gynecol 1989; 161:193.
  109. Hill LM, Guzick D, Rivello D, et al. The transverse cerebellar diameter cannot be used to assess gestational age in the small for gestational age fetus. Obstet Gynecol 1990; 75:329.
  110. Mayden KL, Tortora M, Berkowitz RL, et al. Orbital diameters: a new parameter for prenatal diagnosis and dating. Am J Obstet Gynecol 1982; 144:289.
  111. Hadlock FP, Deter RL, Harrist RB, et al. A date-independent predictor of intrauterine growth retardation: femur length/abdominal circumference ratio. AJR Am J Roentgenol 1983; 141:979.
  112. Hadlock FP, Harrist RB, Shah Y, Park SK. The femur length/head circumference relation in obstetric sonography. J Ultrasound Med 1984; 3:439.
  113. Hadlock FP, Harrist RB, Martinez-Poyer J. Fetal body ratios in second trimester: a useful tool for identifying chromosomal abnormalities? J Ultrasound Med 1992; 11:81.
  114. Hohler CW, Quetel TA. Comparison of ultrasound femur length and biparietal diameter in late pregnancy. Am J Obstet Gynecol 1981; 141:759.
  115. Chinn DH, Bolding DB, Callen PW, et al. Ultrasonographic identification of fetal lower extremity epiphyseal ossification centers. Radiology 1983; 147:815.
  116. Mahony BS, Callen PW, Filly RA. The distal femoral epiphyseal ossification center in the assessment of third-trimester menstrual age: sonographic identification and measurement. Radiology 1985; 155:201.
  117. Mahony BS, Bowie JD, Killam AP, et al. Epiphyseal ossification centers in the assessment of fetal maturity: sonographic correlation with the amniocentesis lung profile. Radiology 1986; 159:521.
  118. Reece EA, Gabrielli S, Degennaro N, Hobbins JC. Dating through pregnancy: a measure of growing up. Obstet Gynecol Surv 1989; 44:544.
  119. Goldstein I, Reece EA, O'Connor TZ, Hobbins JC. Estimating gestational age in the term pregnancy with a model based on multiple indices of fetal maturity. Am J Obstet Gynecol 1989; 161:1235.
  120. Sholl JS, Woo D, Rubin JM, et al. Intrauterine growth retardation risk detection for fetuses of unknown gestational age. Am J Obstet Gynecol 1982; 144:709.
  121. Smith GC, Smith MF, McNay MB, Fleming JE. First-trimester growth and the risk of low birth weight. N Engl J Med 1998; 339:1817.
  122. Henriksen TB, Wilcox AJ, Hedegaard M, Secher NJ. Bias in studies of preterm and postterm delivery due to ultrasound assessment of gestational age. Epidemiology 1995; 6:533.
  123. Morin I, Morin L, Zhang X, et al. Determinants and consequences of discrepancies in menstrual and ultrasonographic gestational age estimates. BJOG 2005; 112:145.
  124. http://www.perinatology.com/calculators/Due-Date.htm (Accessed on March 21, 2014).
  125. https://play.google.com/store/apps/details?id=com.fertilitycoucil.duedateview (Accessed on March 24, 2014).