The Pregnancy Conception Calculator estimates the date of conception based on the expected DUE DATE of the pregnancy, LAST PERIOD date, or ULTRASOUND date.

  1. Due Date: If you know the day your baby is expected to be born, the calculator can work backwards to find out when you likely conceived.
  2. Last Period: If you remember the first day of your last menstrual period, the calculator uses this date. It’s based on the idea that ovulation (when your egg is released and can be fertilized by sperm) usually happens about two weeks after your period starts.
  3. Ultrasound: Sometimes, the date from your ultrasound can help determine when you got pregnant.

Modify the values and click the Calculate button to use

Conception calculator

The calculator also keeps in mind that sperm can live inside your body for about 3-5 days. So, you could have conceived a few days after you had sex. This tool gives you a range of days when conception might have happened.

How to calculate the conception date

There are 3 possible ways to calculate your conception date:

  • Last Menstrual Period (LMP): If you know the first day of your last period, doctors use this to guess when you ovulated and when you might have conceived. It’s usually about 11-21 days after the start of your last period.
  • Due Date: If you’ve been to a doctor and they’ve given you a due date (probably from an ultrasound), you can work backwards to find a range of days when you might have conceived.
  • Ultrasound: This method uses sound waves to create pictures of the inside of your body. It’s super helpful, especially early in pregnancy, to estimate when you got pregnant.

The American College of Obstetricians and Gynecologists recommends that ultrasound-established dates should take preference over menstrual dates when the discrepancy between ultrasound dating and LMP is

  • Greater than 5 days before 9 0/7 weeks of gestation by LMP
  • Greater than 7 days from 9 0/7 weeks to 15 6/7 weeks by LMP
  • Greater than 10 days from 16 0/7 weeks to 21 6/7 weeks by LMP
  • Greater than 14 days from 22 0/7 weeks to 27 6/7 weeks by LMP
  • Greater than 21 days after 28 0/7 weeks by LMP

“Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; they need to be guided by careful consideration of the entire clinical picture and may require closer surveillance, including repeat ultrasonography to ensure appropriate interval growth.” [24].

Why is it hard to know the exact day of conception?

  • Ovulation (when your egg is released) can vary a lot, especially if your periods are irregular.
  • Sperm can live in your body for up to 5 days after sex, so the actual conception could happen days after you had sex.[26,27]

Observed Gestational Age at Delivery

The table below shows the average age of infants delivered in the United States according to the number of fetuses being carried (plurality) [29].

SingletonTwinsTripletsQuadrupletsQuintuplets and
higher-order multiples
Average Gestational Age (weeks)3935323027

The earlier age at delivery of multiple gestations is a reflection of the increased incidence of preterm labor and obstetric intervention for complications such as preeclampsia, abruptio placentae, fetal growth restriction, and increased risk for stillbirth that occurs as the number of fetuses increases [30-33]. The tendency for multiple gestations to be delivered earlier than singleton pregnancies should not be interpreted that multiples should be assigned an earlier estimated due date. A full term pregnancy is considered to be 39 to 40 6/7 weeks regardless of the number fetuses being carried [34].

So, when did I likely get pregnant?

To get a good estimate, think about the first day of your last period, any dates you might have had sex, and any ultrasound dates you have. Remember, these methods give you a range of possible dates – it’s tricky to pinpoint the exact day.

Understanding when you got pregnant can feel like a bit of a puzzle, but these methods can help you get a pretty good idea. Remember, everyone’s body is different, so these are general guidelines. If you’re ever unsure, your doctor is the best person to help you figure it out!

FAQs about Conception Estimator

What does conception actually mean?

Conception is when you become pregnant. It involves two key steps: 1. Fertilization: This is when a sperm from a man meets and joins with an egg from a woman. This usually happens in a tube called the fallopian tube. 2. Implantation: This is when the fertilized egg attaches itself to the wall of your uterus (womb).

What’s the difference between fertilization and conception?

In simple terms, fertilization is just the sperm meeting the egg. Conception includes both fertilization and the moment the fertilized egg implants in your uterus. Some people use these terms interchangeably, but medically, they’re a bit different.

When Did I Conceive?

When exactly did I get pregnant? Many women ask this question when trying to figure out the exact day their pregnancy began. Often, gestational age, or the age of the baby, is calculated from the first day of the mother’s last menstrual period. Biologically, the baby was not conceived until ovulation and the fertilization of the egg, which usually happens at least 10 days after the first day of the mother’s last menstrual period. Because there are many factors surrounding conception, it is difficult to pinpoint an exact date of conception. This calculator helps to estimate the date range of real conception and a possible range of days during which sexual intercourse might have led to conception. There are a few methods that can be used to estimate the conception dates.


1. Chervenak FA, Skupski DW, Romero R, et al: How accurate is fetal biometry in the assessment of fetal age?. Am J Obstet Gynecol 1998; 178:678.PMID:9579429
2. Mongelli M, Wilcox M, Gardosi J.: Estimating the date of confinement: ultrasonographic biometery versus certain menstrual dates. Am J Obstet Gynecol 1996; 174:278.PMID:8572021
3. Savitz DA, Terry Jr JW, Dole N, et al: Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol 2002; 187:1660.PMID:12501080
4. ACOG Practice Bulletin. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997). Management of Postterm Pregnancy.Obstet Gynecol. 2004;104:639-46.PMID:15339790
5. Kramer MS, McLean FH, Boyd ME, Usher RH: The validity of gestational age estimation by menstrual dating in term, preterm and postterm gestations. JAMA 1988; 260:3306. PMID:3054193
6. 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 Dec;187(6):1660-6. PMID:12501080
7. Wilcox M, Gardosi J, Mongelli M, et al. Birth weight from pregnancies dated by ultrasonography in a multicultural British population. BMJ. Sep 4 1993;307(6904):588-91.PMID:8401014
8. Ultrasonography in Pregnancy. ACOG Practice Bulletin No. 98. American College of Obstetricians and Gynecologists. Obstet Gynecol 2008;112: 1419–44.
9. 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 Feb;73(2):243-9.PMID: 2643067
10. Henderson SO, Mallon WK.Trauma in pregnancy. Emerg Med Clin North Am. 1998 Feb;16(1):209-28.PMID: 9496322
11. Hay DL, Lopata A.Chorionic gonadotropin secretion by human embryos in vitro. J Clin Endocrinol Metab. 1988 Dec;67(6):1322-4.PMID: 2461389
12. Wilcox AJ, et al. Time of implantation of the conceptus and loss of pregnancy.N Engl J Med. 1999 Jun 10;340(23):1796-9.PMID: 10362823
13. Lohstroh P, et al. Daily immunoactive and bioactive human chorionic gonadotropin profiles in periimplantation urine samples. Biol Reprod. 2006 Jul;75(1):24-33. PMID: 16525035
14. Cervical insufficiency. ACOG Practice Bulletin No. 48. The American College of Obstetricians and Gynecologists. Obstet Gynecol 2003;102:1091–9. PMID: 15124633
15. Berghella V, et al., Does transvaginal sonographic measurement of cervical length before 14 weeks predict preterm delivery in high-risk pregnancies? Ultrasound Obstet Gynecol 2003;21:140-144. PMID: 12601834
16. Ultrasonography in Pregnancy. ACOG Practice Bulletin No. 101. American College of Obstetricians and Gynecologists. Obstet Gynecol 2009;113: 451–61. PMID: 19155920
17. ACOG practice bulletin. Prevention of Rh D alloimmunization. Number 4, May 1999 (replaces educational bulletin Number 147, October 1990). Clinical management guidelines for obstetrician-gynecologists. American College of Obstetrics and Gynecology. Int J Gynaecol Obstet. 1999 Jul;66(1):63-70. PMID: 10458556
18. Fetal Lung Maturity. ACOG Practice Bulletin No. 97. American College of Obstetricians and Gynecologists. Obstet Gynecol 2008;112: 717–26PMID: 18757686
19. ACOG practice bulletin. Antepartum fetal surveillance. Number 9, October 1999 (replaces Technical Bulletin Number 188, January 1994). Clinical management guidelines for obstetrician-gynecologists. Int J Gynaecol Obstet. 2000 Feb;68(2):175-85. PMID: 10717828
20. Institute for Clinical systems Improvement. Routine Prenatal Care. 13th ed. August 2009. Available at:
21. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 101: Ultrasonography in pregnancy. Obstet Gynecol. 2009 Feb;113(2 Pt 1):451-61. PMID: 19155920
22.Morin L, Lim K. Ultrasound in twin pregnancies. J Obstet Gynaecol Can. 2011 Jun;33(6):643-56. PMID: 21846456
23. Sperling L, Tabor A.Twin pregnancy: the role of ultrasound in management.Acta Obstet Gynecol Scand. 2001 Apr;80(4):287-99. PMID: 11264601
24. Method for estimating due date. Committee Opinion No. 611. Ameri-can College of Obstetricians and Gynecologists. Obstet Gynecol 2014;124:863–6. PMID: 25244460
26. Allen J Wilcox, David Dunson, and Donna Day Baird. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study BMJ 2000; 321: 1259-1262.  PMID: 11082086
27. Wilcox AJ, Weinberg CR, Baird DD.Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995 Dec 7;333(23):1517-21.  PMID:7477165 28. Determination of Gestational Age by Ultrasound No. 303, February 2014 Society of Obstetricians and Gynaecologists of Canada.J Obstet Gynaecol Can 2014;36(2):171–181
29. Martin JA, et al., Births: final data for 2009. Natl Vital Stat Rep. 2011 Nov 3;60(1):1-70. PMID: 22670489
30. Multifetal gestations: twin, triplet, and higher-order multifetal pregnancies. Practice Bulletin No. 144. American College of Obstetricians and Gynecologists. Obstet Gynecol 2014;123:1118–32. PMID: 24785876
31. Page JM, The risk of stillbirth and infant death by each additional week of expectant management in twin pregnancies. Am J Obstet Gynecol. 2015 May;212(5):630.e1-7. doi: 10.1016/j.ajog.2015.03.033. Epub 2015 Mar 19. PMID: 25797235
32. Burgess JL, Unal ER, Nietert PJ, Newman RB.Risk of late-preterm stillbirth and neonatal morbidity for monochorionic and dichorionic twins. Am J Obstet Gynecol. 2014 Jun;210(6):578.e1-9. doi: 10.1016/j.ajog.2014.03.003. Epub 2014 Mar 5. PMID: 24607757
33. Kahn B, et. al., Prospective risk of fetal death in singleton, twin, and triplet gestations: implications for practice. Obstet Gynecol. 2003 Oct;102(4):685-92. PMID: 14550996
34. Definition of term pregnancy. Committee Opinion No. 579. American College of Obstetricians and Gynecologists. Obstet Gynecol 2013;122:1139–40 PMID: 24150030

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