After your baby's birth, you might be wondering the date of his conception. Your due date was probably based on your last menstrual period---unless you had an early ultrasound---and that doesn't tell you when the baby was actually conceived. In addition to using your last menstrual period to determine conception dates, you can calculate your conception date using the Prem method.
Use the Prem method to determine your conception date if you have irregular, long or short cycles. Even if you have regular cycles, the Prem method should give you a reliable result. According to John and Sheila Kippley, authors of "The Art of Natural Family Planning," the Prem method was developed by Dr. Konald A. Prem, professor of Obstetrics and Gynecology at the University of Minnesota School of Medicine.
Subtract nine months from the birthday in question. For instance, if your child's birthday is November 20, you'll get February 20.
Add seven days. In this example, you'll end up with February 27. This date would represent an approximate date for your full thermal shift, which occurs immediately after ovulation, the Kippleys say. It's likely, then, that you conceived a baby born on November 20 right around or slightly before February 27.
You can also use the date of your last menstrual period if you have a regular, 28-day cycle. While this doesn't necessarily use your child's birthdate, it's a quick way to determine a possible day for ovulation. Using your last menstrual period won't get you a specific date, but rather a range of possible dates.
Determine the first day of your last menstrual period before you got pregnant with your baby.
Add 11 to 21 days, the American Pregnancy Association says. So, if your last menstrual period was April 2, you ovulated sometime between April 13 and 23.
Keep in mind that when you were pregnant, your due date was calculated from your last menstrual period and not your conception date. If you underwent fertility treatment, such as artificial insemination or in vitro fertilisation, you should go by that date as your date of conception, the American Pregnancy Association says. Sperm can live as long as three to five days in your vaginal canal. Look at a full three to five days before your estimated ovulation date to determine when intercourse led to conception. Even if you don't have intercourse on the day of or the day before you ovulate, you can still get pregnant from lingering sperm.
If your baby was born late or early, or even if your labour was induced, you will not be able to get an accurate conception date based on your baby's birthday.