Are Baby Due Date Calculators Accurate?

There are several ways that a due date calendar will calculate your due date. This may depend on the information that you have or a procedure that was done. When looking at due date calculators, consider the following methods of calculation:

• First day of your last period: When using this method, you’ll enter the start date of your last period. The system will calculate 40 weeks from that day (280 days). This is how many doctors establish your due date. Using this same method, you can subtract three months from the start day of your last period and then add seven days. This is just another way to do the same calculation.
• Conception date: If you know your conception date because you were using an ovulation predictor kit or otherwise tracking ovulation, you can use this method. You’ll add 266 days to the conception date to get the due date estimation.
• IVF transfer date: Using the in vitro fertilization calculation, you’ll use the day that you transferred the embryo. A Day 3 embryo transfer adds 263 days while a Day 5 embryo transfer adds 261 days.
• Ultrasound: This is used if you don’t have any of the above details for one of the other calculators. By tracking the baby’s heartbeat or fetal movement, you can get an estimate of the due date. When using the baby’s heartbeat milestone, your doctor knows that this starts at week 9 or 10 of pregnancy, so they add 31 weeks. When you experience the first fetal movement will be around week 18 to 22, so you’d add 22 weeks to the date. “It’s generally accepted that earlier scans, done at about 12 weeks, are most accurate for dating pregnancy,” according to what David Beatty, MRCGP, shared with Mom.com.

Pregnancy calculators are not always accurate. I can attest to this, having had to bear an extra two weeks in the summer heat of California waiting for my son to pop out.

It’s not uncommon for due dates to be off by a week or two. In some cases, your doctor will change the due date with new information that presents itself. Reasons for changes could be because you have irregular periods, or your first ultrasound wasn’t until the second trimester. Doctors will also change the due date if the fundal height is abnormal, or your alpha-fetoprotein (AFP) levels are outside the expected range.

It’s best to use a due date calculator as a guide and work with your doctor to make sure everything is on track. Moms will tell you that their babies seldom arrived on their due date, but that the calculators get pretty close. “While only one of my four children was actually born on his due date, the calculators have pretty consistently provided dates that match my doctor’s projection,” mom Alexandra Fung told Mom.com.

As many as 11% of babies are born preterm — that’s before week 37. Most preterm births are unanticipated, though 30% are due to preterm rupture of membranes and 20% are medically induced. The medically induced group could be due to conditions such as preeclampsia, where signs can include headaches, vision problems, severe heartburn, pain below the ribs, nausea, and edema.

It’s thought that some 10% of babies are born post-term. While there is little scientific evidence to discern that a baby will be born late, many moms over the years have noticed a few different things that might indicate that your baby will be late. Here are some signs that you might not deliver on time:

• First-time mother
• Baby hasn’t dropped
• No changes in the cervix
• Unknown inception date thus inaccurate due date
• Inactive baby

While it is quite common for babies to be born after their due date, don’t hesitate to talk to your doctor to make sure everything is normal with your pregnancy. Your doctor may decide to induce to prevent potential problems with the birth and to protect both you and your baby.