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.
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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.