Gestational Length Calculator

This calculator estimates pregnancy duration based on scientific research on factors that influence gestational length. Enter your information below for a personalized estimate.

Essential Information

Maternal Factors

Pregnancy Factors

Previous Pregnancies

If you've been pregnant before, enter the length in days of each pregnancy from implantation to birth (or estimate from ovulation to birth).

Estimated Gestational Length

From implantation to birth: 268 days (38.3 weeks)
Likely range: 258-278 days (36.9-39.7 weeks)
Implantation success rate:
High (>85%)

Scientific Basis

This calculator is based on research findings that show pregnancy duration from implantation to birth has a median of 268 days (38 weeks and 2 days), but can naturally vary by as much as 37 days among healthy pregnancies.

Notably:

Note: This calculator makes estimates based on my assumptions. I weighted the factors entirely based on intuition, so it might be right, but it is non-rigorous. I also made some assumptions about the distribution of implantation times, which may not be accurate.

References

  1. Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999;340(23):1796-1799.
  2. Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848-2855.
  3. Oberg AS, Frisell T, Svensson AC, Iliadou AN. Maternal and fetal genetic contributions to postterm birth: familial clustering in a population-based sample of 475,429 Swedish births. Am J Epidemiol. 2013;177(6):531-537.
  4. Halloran DR, Cheng YW, Wall TC, Macones GA, Caughey AB. Effect of maternal weight on postterm delivery. J Perinatol. 2012;32(2):85-90.
  5. Divon MY, Ferber A, Nisell H, Westgren M. Male gender predisposes to prolongation of pregnancy. Am J Obstet Gynecol. 2002;187(4):1081-1083.
  6. Mogren I, Stenlund H, Högberg U. Recurrence of prolonged pregnancy. Int J Epidemiol. 1999;28(2):253-257.
  7. Olesen AW, Basso O, Olsen J. Risk of recurrence of prolonged pregnancy. BMJ. 2003;326(7387):476.
  8. Johnsen SL, Wilsgaard T, Rasmussen S, Sollien R, Kiserud T. Longitudinal reference charts for growth of the fetal head, abdomen and femur. Eur J Obstet Gynecol Reprod Biol. 2008;127(2):172-185.
  9. Margerison-Zilko CE, Catalano R, Hubbard A, Ahern J. Maternal exposure to unexpected economic contraction and birth weight for gestational age. Epidemiology. 2015;26(5):622-630.
  10. Taipale P, Hiilesmaa V. Predicting delivery date by ultrasound and last menstrual period in early gestation. Obstet Gynecol. 2001;97(2):189-194.
  11. Norman RJ, Menabawey M, Lowings C, Buck RH, Chard T. Relationship between blood and urine concentrations of intact human chorionic gonadotropin and its free subunits in early pregnancy. Obstet Gynecol. 1987;69(4):590-593.
  12. Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH. Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol. 2002;187(6):1660-1666.
  13. ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
  14. Baird DD, McConnaughey DR, Weinberg CR, et al. Application of a method for estimating day of ovulation using urinary estrogen and progesterone metabolites. Epidemiology. 1995;6(5):547-550.
  15. Armstrong EG, Ehrlich PH, Birken S, et al. Use of a highly sensitive and specific immunoradiometric assay for detection of human chorionic gonadotropin in urine of normal, nonpregnant, and pregnant individuals. J Clin Endocrinol Metab. 1984;59(5):867-874.
  16. Canfield RE, Ross GT. A new reference preparation of human chorionic gonadotropin and its subunits. Bull World Health Organ. 1976;54(4):463-472.