16 weeks pregnant — due date & trimester
Here's the estimated due date and trimester for someone 16 weeks pregnant today.
This is a general estimate, not a substitute for advice from a doctor or midwife. Use the full due date calculator with your exact dates.
About the due date calculator
This calculator estimates your baby's due date using one of two common methods: the first day of your last menstrual period (LMP), or a known conception date. The LMP method uses Naegele's rule, the standard formula most clinicians use as a starting point — it adds 280 days (40 weeks) to the first day of your last period, since pregnancy is conventionally counted from that date even though conception typically happens roughly two weeks later. The conception-date method instead adds 266 days (38 weeks) directly to the day you conceived, which lands on the same estimated due date when a 28-day cycle and typical ovulation timing are assumed.
Alongside your due date, the calculator also shows how far along you are right now — in completed weeks and days — and which trimester that falls into, using the widely used breakdown of weeks 1–13 for the first trimester, 14–27 for the second, and 28–40 for the third. These numbers update automatically based on today's date, so you can bookmark the page and check back later in your pregnancy without re-entering anything.
It's worth understanding what "40 weeks" really means: it's an average, not a guarantee. Full-term pregnancies routinely deliver anywhere from about 37 to 42 weeks, and research suggests only a small fraction of babies actually arrive on their precise calculated due date. Due dates are most useful as a planning anchor — for scheduling prenatal appointments, tracking fetal growth milestones, and knowing when a pregnancy is considered full term — rather than as a firm prediction of birth day.
The accuracy of any due date estimate also depends heavily on how regular your cycle is. Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14; if your cycles typically run longer or shorter than that, the calculated date can be off by several days in either direction. This is one of the reasons an early ultrasound — which measures the size of the fetus directly — is often considered more precise than a date-based calculation, especially for people with irregular cycles.
Disclaimer: this tool provides a general estimate based on average cycle lengths and standard obstetric formulas. It is not medical advice and should not replace guidance from a doctor, midwife, or other qualified healthcare provider, who can factor in ultrasound measurements, cycle history, and other individual details that this calculator cannot see.
Due date calculator questions
How due dates, pregnancy weeks, and trimesters are calculated.
How is my due date calculated?
From your last menstrual period (LMP), the calculator adds 280 days (40 weeks) using Naegele's rule. From a known conception date, it adds 266 days (38 weeks) instead. Both methods assume a fairly typical cycle and generally land on the same estimated date.
Why does the LMP method add 280 days when pregnancy is only 38 weeks long?
Pregnancy is conventionally dated from the first day of your last period, not from conception, even though conception usually happens about two weeks later. That's why the LMP calculation adds an extra 14 days compared to the conception-based calculation — they're measuring from different starting points but aiming at the same due date.
How accurate is a due date calculator?
It's a helpful estimate, not a guarantee. Only a small percentage of babies are born on their exact calculated due date, and full-term deliveries commonly happen anywhere from about 37 to 42 weeks. An ultrasound in the first trimester generally gives a more precise estimate than date-based methods alone.
What trimester am I in?
This calculator uses the common breakdown of weeks 1–13 for the first trimester, weeks 14–27 for the second, and weeks 28–40 for the third, based on how many weeks along you are today.
What if I don't know my last menstrual period date?
Switch to the conception-date mode if you know roughly when you conceived — for example, from ovulation tracking or a specific event. If neither date is known, an early ultrasound is the most reliable way to establish a due date.
Does this calculator account for irregular cycles?
No — it assumes a standard 28-day cycle with ovulation around day 14, which is the same assumption behind Naegele's rule. If your cycles are consistently longer or shorter, your actual due date may fall a few days earlier or later than shown here.
How many weeks pregnant am I today?
The calculator compares today's date to your last menstrual period (or the equivalent estimated LMP if you entered a conception date instead) and shows the result in completed weeks and days.
Is this due date calculator a substitute for medical advice?
No. This tool gives a general estimate based on average cycle lengths and standard formulas. It isn't a substitute for care from a doctor or midwife, who can incorporate ultrasound dating, your specific cycle history, and other individual factors this calculator can't account for.