If you have just recently learned that you are pregnant, the first question on your mind might be just how pregnant you actually are and when you can expect to be giving birth to your baby, known as the "due date" for your pregnancy. In human beings, pregnancy lasts an average of about 40 weeks from the start of the woman's last menstrual period--but there are a number of ways to try to date the pregnancy.
Instructions
1. Get out a calendar and count 40 weeks from the first day of your last known menstrual period. If you know your last menstrual period, this will be how your doctor calculates your due date, although he will most likely use a wheel that automatically calculates this rather than count weeks on a calendar. This method works best if you have regular 28-day menstrual cycles. Women ovulate about 2 weeks before their menstrual periods begin, so if you have a longer cycle or an irregular menstrual cycle, that affects your ovulation date and this method will not be accurate (given that your pregnancy technically begins after ovulation and the fertilization of the egg).
2. Count 9 months on the calendar from the date of your last menstrual period and then add 2 more weeks; 40 weeks of pregnancy translates to about 9 1/2 calendar months (or 9 months from conception if you have 28-day cycles with ovulation in the middle of your menstrual cycle). This is a more rough estimate of when to expect your baby, but it is easier than counting weeks.
3. Count 38 weeks from your ovulation date if you were actively trying to conceive and happen to know which day you ovulated. This is the most accurate means of calculating your due date, but it requires that you actually know your ovulation date--such as if you were using ovulation predictor strips or temperature charting during your menstrual cycle. But even if you have irregular cycles, if you know your ovulation date, this method gives you an accurate estimate of your due date.
4. Count 38 weeks from the date that you had sexual intercourse if you know for certain which incident of sexual intercourse resulted in the pregnancy (such as if you have only had sex once in the last several months). Sperm can live inside a woman's body for 5 to 7 days, so this method would give you an estimated due date within 5 to 7 days of accuracy--but only if you are certain about the date of the sex that caused the pregnancy.
5. Visit your doctor for an early ultrasound. A transvaginal ultrasound in early pregnancy allows the doctor to measure the gestational sac and early developing baby to correspond to dates of development and give you a due date within 7 days of accuracy. Ultrasound becomes less accurate as a tool for calculating the due date as the pregnancy progresses, because the margins of error become wider (since babies grow at different rates). Your doctor will probably use this method to calculate your due date if you do not know the date of your last menstrual period or the likely date of conception.
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