Use the drop-down menu to calculate your due date or current week of pregnancy.
This calculator allows you to calculate the following:
Last menstrual period (LMP): The LMP calculation uses the standard method of the first day of your last menstrual period plus 280 days to calculate the due date.
Ultrasound due date: If a due date has been calculated from an ultrasound in the first trimester, the reverse calculation method is preferred. This calculates the current gestational age and gives an estimated date of LMP.
Conception date: If conception is known (eg, date of insemination, ovulation or egg retrieval), then the due date can be calculated by adding 266 days.
Gestation at a future or past date: Use to calculate gestation in the future or past. First, calculate current gestation using one of the other methods, then go to this section and choose past or future date.
Date at future or past gestation: Use to calculate the date at a specific gestation (enter in weeks). First, calculate current gestation using one of the other methods, then go to this section and choose a gestational age.
Enter the LMP ....or click on the calendar to select
....or click on the EDC
....or choose any day to calculate the gestation on that day
alternatively, this function will tell you the adjusted gestational age today, based on GA at birth
Enter gestational age at birth weeks and days
Enter the bith date
Version 1.3 October 2009
Dr Girvan Malcolm
By knowing the first day of your last period, or your last menstruation day, you can figure out how far along you are in your pregnancy and what month you are in. With this information, you can also estimate your due date.
Through ultrasound, the doctor can also determine how far along you are. Fetal and uterine measurements can accurately relay the baby's age and estimate the delivery date.
Enter the first day of your last period below to calculate how far along you are in weeks and months, and when you can expect your baby to be born:
What if i don't know the date of my last period?
Sometimes it is not possible to know when your last period was, and in these cases, the doctor can order a dating ultrasound. My measuring uterine height, the ultrasound technician or doctor can calculate how far along you are as well as your estimated due date. Oftentimes, an ultrasound and a blood test to check beta-HCG levels are ordered together to determine dating.
1. How to calculate in weeks
To calculate how many weeks you are, start by taking note of the first day of your last period on a calendar, or your LMD (last menstruation day). Every 7 days from that first day counts as 1 week in your pregnancy.
For example, if the first day of the last period was March 11 and you recently had a positive pregnancy test, your first day of pregnancy is March 11 (and not the day of conception, or when you had sex). If March 11 was day 1, then March 18 (a week later) will mark week 1 of your pregnancy. April 16, for example is calculated as 1 week and 1 day of your pregnancy, and you will have been in your second month.
Calculation is done this way because the exact fertilization day is difficult to determine. Sperm cells can survive in the woman's body for up to 7 days, and can fertilize the egg and initiate the pregnancy on any of these days.
2. How to calculate in months
To determine how far along you are, you can reference our table below
TrimesterMonthWeeks Included1st trimester1 monthup to 4 and a half weeks of pregnancy1st trimester2 months
4 and a half weeks to 9 weeks
1st trimester3 months10 to 13 and a half weeks of pregnancy
2nd trimester4 months13 and a half weeks to 18 weeks of pregnancy
19 to 22 and a half weeks of pregnancy
2nd trimester6 months23 to 27 weeks of pregnancy
3rd trimester7 months28 to 31 and a half weeks of pregnancy
3rd trimester8 months32 to 36 weeks of pregnancy
3rd trimester9 months37 to 42 weeks of pregnancy
Usually a pregnancy lasts 40 weeks, but the baby can be born between 39 and 41 weeks without any problems. If labor does not start spontaneously until the 41-week mark, the doctor may choose to induce labor with IV oxytocin.
How to calculate the estimated due date
To calculate the probable due date, which should be around 40 weeks after your LMD, add 7 days to your LMD, and then count 3 months back and add one year.
For example, if the LMD was March 11, 2021, add 7 days, (March 18, 2021), then subtract 3 months (December 18, 2020) and add 1 year (December 18, 2021). In this case, the estimated due date is December 18, 2018.
This calculation is not because the baby can be born between 37 and 42 weeks of pregnancy, however, it allows the parents and health care team to prepare for delivery.