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Pregnancy due date


Related terms
Author information
Charts and calculators

Related Terms
  • Baby, conception, fetus, gestational age, in vitro fertilization, menstrual period, menstrual cycle, pregnancy, pregnant, ultrasound.

  • Based on a woman's last menstrual period, the estimated due date is 40 weeks from the first day of the last period. This is just an estimate since only about 5% of babies are born on their estimated due date.
  • When the egg is available for fertilization and 350 million sperm are released inside or near the woman's vaginal area, about 200 sperm will reach the egg. Of those 200 sperm, only one will actually penetrate the egg, thus causing conception. Once a woman is pregnant, regular checkups by a doctor, certified nurse-midwife or other health care professional during the course of pregnancy are recommended. The goal of prenatal care is to monitor the progress of a pregnancy and to identify potential problems before they become serious for either the mother or the baby.
  • Gestational age, or how far along the fetus is, can be approximated by the beginning of the mother's last period. Babies are usually born at 40 weeks, but are considered full-term from weeks 37 to 42. The birth of an infant before 37 completed weeks of gestation (at least 3 weeks before the "due date") is considered pre-term.

Theory / Evidence
  • Pregnancy due date calculators rely on estimates using the available information of average cycle length. There is no way to predict the exact due date, as each pregnancy is unique; however, pregnancy due date calculators can give you a general estimate of conception date, gestational age and expected due date.
  • By using the first day of a woman's last menstrual cycle, a woman can figure out when she most likely conceived, how old her baby currently is, and when her baby's birth is expected. Due date calculators rely on using the knowledge of women's individual cycle lengths (usually between 22 and 44 days with 28 being the average cycle length). If the cycle length is unknown, use 28 days as a default, which is the average for most women.
  • If the mother has a regular period and knows the first day of her last menstrual period, gestational age can be calculated from this date. Gestational age is calculated from the first day of the mother's last menstrual period and not from the date of conception.
  • In woman with a regular period, conception typically occurs about two weeks after the first day of the last period. Most women do not know the exact date of conception, and their conception date is merely an estimate based on the first day of their last period.
  • Special cases: Women who undergo special procedures such as artificial insemination or in vitro fertilization typically know the exact date of conception.
  • For women who have irregular menstrual periods or women who cannot remember the first day of their last menstrual period, it can be difficult to determine gestational age and due date using this method. In these cases, an ultrasound exam is often required to determine gestational age.
  • Other methods: It is difficult to determine the gestational age (and due date) in some cases because the baby is unusually large or small. Also, in some cases the size of the uterus in early pregnancy or the height of the uterus in later pregnancy does not match the first day of the last menstrual period. In these cases as well, it is difficult to obtain an accurate gestational age. Ultrasound is typically used and the baby can be measured as early as 5 or 6 weeks after the mother's last menstrual period. Measuring the baby using ultrasound is most accurate in early pregnancy. It becomes less accurate later in pregnancy. The best time to estimate gestational age using ultrasound is between the 8th and 18th weeks of pregnancy. The most accurate way to determine gestational age is using the first day of the woman's last menstrual period and confirming this gestational age with the measurement from an ultrasound exam.
  • Pre-term birth is often characterized as "very pre-term" (less than 32 weeks gestation) and moderately pre-term (32-36 weeks gestation). Pregnancy due date calculators typically cannot foresee pre-term birth, but some women may be at a greater risk for preterm birth than others. According to Martin et al. (2003) African American women have 1.6 times the risk of delivering a pre-term infant compared to white women; the excess risk for delivering a very preterm infant is even greater for black women who have 2.5 times the risk compared to white women. Unfortunately, the risk of preterm delivery has been increasing in the U.S. since the mid-1990s. A 2004 report by Hamilton et al. showed that in 2003 the rate of preterm births rose to 12.3% compared to 12.1% in 2002. The reasons for the increase in preterm births remain unclear.

Author information
  • This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (

  1. American Pregnancy
  2. Hamilton,B.E., Martin,J.A. and Sutton,P.D. Births: Preliminary Data for 2003. National Vital Statistics Reports. 2004; 53(9)1-18.
  3. March of Dimes. 8 May 2006.
  4. Martin,J.A, Hamilton,B.E., Sutton,P.D.Ventura, S.J, Menacker, F. and Munson,M.L. Births: Final data for 2002. National Vital Statistics Reports. 2003; 52(10)1-114.
  5. U.S. Centers for Disease Control and Prevention.

Charts and calculators
  • In order to calculate your approximate due date, answer the following questions:
  • What was the first day of your last menstrual period (first day of bleeding)
  • What is your average cycle length (from first day of your period to the first day of your next period)?
  • Estimated fertility window: First day of last menstrual period + 10 days through first day of last menstrual period + 20 days. So, if your period started on May 1, then your fertility window would be: May 11 through May 21
  • Estimated conception date: First day of last menstrual period + 15 days. So, if your period started on May 1, then your estimated conception date would be: May 16
  • Estimated gestational age: First day of last menstrual period + elapsed days. So, if your period started on May 1, and today's date is July 1, then your estimated gestational age (how far along in your pregnancy) is approximately 8 weeks, 4 days (60 days).
  • Estimated due date: First day of last menstrual period + 40 weeks. So, if your period started on May 1, your estimated due date would be February 5.

Copyright © 2011 Natural Standard (

The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.

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