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
| Trimester | Month | Weeks Included |
|---|---|---|
| 1st trimester | 1 month | up to 4 and a half weeks of pregnancy |
| 1st trimester | 2 months | 4 and a half weeks to 9 weeks |
| 1st trimester | 3 months | 10 to 13 and a half weeks of pregnancy |
| 2nd trimester | 4 months | 13 and a half weeks to 18 weeks of pregnancy |
| 2nd trimester | 5 months | 19 to 22 and a half weeks of pregnancy |
| 2nd trimester | 6 months | 23 to 27 weeks of pregnancy |
| 3rd trimester | 7 months | 28 to 31 and a half weeks of pregnancy |
| 3rd trimester | 8 months | 32 to 36 weeks of pregnancy |
| 3rd trimester | 9 months | 37 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.
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