What This Calculator Does
The Time Until Due Date Calculator tells you two key things about a pregnancy: how many weeks pregnant you are right now (your gestational age) and how many days remain until your estimated due date. It works from the first day of your last menstrual period (LMP), using the standard Naegele-based estimate that a full-term pregnancy lasts about 280 days (40 weeks) from the LMP.
How to Use It
Enter the first day of your last menstrual period, confirm today's date (it defaults to today), and optionally adjust your average cycle length. The calculator then shows your current gestational age in weeks and days, the number of days left until your due date, and the total days you have been pregnant.
The Formula Explained
Gestational age is simply the time elapsed since the LMP: \(\text{weeks} = (\text{today} - \text{LMP}) \div 7\). The estimated due date sits 280 days after the LMP. If your typical cycle is not 28 days, ovulation shifts, so we adjust the due date by \((\text{cycle} - 28)\) days. Days remaining is then:
$$\text{Days Remaining} = (\text{LMP} + 280 + \text{cycle adjustment}) - \text{today}$$
Worked Example
Suppose your LMP was 2024-01-01 and today is 2024-04-01 with a 28-day cycle. That is 91 days since LMP, or 13 weeks exactly:
$$\text{Weeks Pregnant} = \frac{91}{7} = 13$$
The due date is January 1 + 280 days = October 7, 2024. From April 1, that leaves 189 days remaining.
FAQ
Is this medically exact? No. It is an estimate. Only about 4% of babies arrive on the exact due date; your healthcare provider's ultrasound dating is more precise.
Why does cycle length matter? A longer cycle usually means later ovulation, which pushes the due date slightly later than the standard 280-day rule.
What if I don't know my LMP? Use the date your provider gave you, or count back from a known ultrasound gestational age.
