To calculate your due date after IUI, take the date of your insemination, subtract 14 days, and then add 280 days (40 weeks). This works because IUI happens right around ovulation, giving you a more precise starting point than most pregnancies have. The result is your estimated due date, or EDD. A simpler shortcut: just add 266 days (38 weeks) directly to your IUI date.
Why IUI Gives You a More Accurate Due Date
Standard pregnancy dating assumes a 28-day menstrual cycle with ovulation on day 14. That’s a rough estimate for anyone, and roughly half of women don’t accurately recall the first day of their last menstrual period (LMP). With IUI, you don’t have to guess. You know the exact date of insemination, and because the procedure is timed to coincide with ovulation, you have a reliable conception date.
This matters because the conventional 280-day (40-week) countdown starts from the first day of your LMP, which is about two weeks before conception actually happens. When you know your IUI date, you’re working with a fixed point that eliminates the guesswork around cycle length, ovulation timing, and memory.
The Step-by-Step Calculation
There are two ways to get to the same answer. Use whichever feels easier.
Method 1: Mimic the LMP formula. Subtract 14 days from your IUI date to create a “theoretical LMP.” Then add 280 days. For example, if your IUI was on March 20, your theoretical LMP would be March 6. Add 280 days and your due date lands on December 11.
Method 2: Count forward from insemination. Add 266 days (38 weeks) directly to the date of your IUI. Using the same example, March 20 plus 266 days also gives you December 11. This method skips the extra step and gets you to the same result because 280 minus 14 equals 266.
Both methods assume that conception happened on or very close to the day of insemination. Since IUI is specifically timed to place sperm when an egg is available, this is a safe assumption.
How the Trigger Shot Factors In
If your cycle included an hCG trigger shot, your IUI was likely scheduled about 36 hours after the injection. That’s the window when ovulation typically occurs. The trigger shot gives your fertility team precise control over timing, which is why the insemination is usually booked for the second morning after the injection.
For due date purposes, the IUI date itself is what you use, not the date of the trigger shot. The insemination is timed to match ovulation as closely as possible, so the IUI date serves as your best proxy for conception day. If you had two IUIs on consecutive days (sometimes called back-to-back IUIs), use the date of the first one.
What Happens When Your Ultrasound Disagrees
At your first ultrasound, typically between 6 and 9 weeks, the technician will measure the embryo from head to rump and estimate gestational age. In most natural pregnancies, this measurement can override a due date based on LMP if there’s a big enough discrepancy. With IUI, the situation is different.
ACOG guidelines state that when pregnancy results from assisted reproductive technology, the treatment-derived gestational age should be used to assign the due date. While IUI is a less complex procedure than IVF, the same principle applies: you have a known conception date, which is more reliable than an ultrasound estimate that comes with its own margin of error. In practice, your provider will likely keep the IUI-based due date unless the ultrasound shows a significant difference that suggests a dating issue.
How Your Provider Records Gestational Age
One detail that confuses many IUI patients is how far along they’re told they are at their first appointment. Gestational age is always counted from the theoretical LMP, not from conception. So on the day of your IUI, you’re already considered about 2 weeks pregnant by medical convention, even though fertilization is just happening.
This means if you get a positive pregnancy test 14 days after your IUI, your chart will say you’re 4 weeks pregnant. That’s not an error. It’s the standard system, and every milestone (first heartbeat, anatomy scan, full term at 39 weeks) is measured on this same timeline. Your actual embryo is always about two weeks younger than your “gestational age” suggests.
A Quick Reference Example
Here’s a complete walkthrough. Say your IUI was performed on June 10.
- Theoretical LMP: June 10 minus 14 days = May 27
- Due date: May 27 plus 280 days = March 3 of the following year
- Check: June 10 plus 266 days = also March 3
- Gestational age on June 10: 2 weeks, 0 days
- Gestational age on positive test (around June 24): 4 weeks, 0 days
Keep in mind that a due date is an estimate, not an appointment. Full-term pregnancy spans from 39 weeks through 40 weeks and 6 days. Most babies arrive within a two-week window on either side of the EDD. Having a precise IUI date doesn’t change the natural variability in when labor begins, but it does give your care team a reliable anchor for tracking growth and planning your care throughout pregnancy.