Question 1

How accurate are due date calculators?

Accepted Answer

Due date calculators based on Naegele's rule are accurate to within 1-2 weeks for most pregnancies. Only about 5% of babies are born on their exact due date. Most arrive within a 2-week window before or after the estimated date. An early ultrasound (before 12 weeks) can refine the estimate to within 5-7 days.

Question 2

What is Naegele's rule?

Accepted Answer

Naegele's rule is the standard method used by healthcare providers worldwide to estimate pregnancy due dates. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle and that ovulation occurred on day 14. For non-standard cycle lengths, the formula is adjusted by adding or subtracting the difference from 28.

Question 3

Does cycle length affect my due date?

Accepted Answer

Yes. Women with longer cycles tend to ovulate later, which pushes the due date forward. For a 35-day cycle, your due date shifts about 7 days later than the standard calculation. Conversely, a 24-day cycle means ovulation likely occurred 4 days earlier, bringing the due date forward by 4 days.

Question 4

What if I don't know my last menstrual period date?

Accepted Answer

If you're unsure of your LMP, your healthcare provider can estimate gestational age using an early ultrasound measurement (crown-rump length). This is most accurate in the first trimester. You can also estimate by working backward from your conception date — add 266 days (38 weeks) from the date of conception.

Question 5

What are the three trimesters of pregnancy?

Accepted Answer

The first trimester spans weeks 1-12 and is when all major organs begin forming. The second trimester covers weeks 13-26 — you'll feel the baby move and can learn the gender. The third trimester (weeks 27-40) is when the baby gains most of its weight and prepares for birth. Each trimester brings different symptoms, tests, and milestones.

Question 6

When should I see a doctor after getting a positive test?

Accepted Answer

Most healthcare providers recommend scheduling your first prenatal appointment between weeks 6-8 of pregnancy (2-4 weeks after a missed period). This first visit typically includes a confirmation of pregnancy, dating ultrasound, blood work, and a review of your medical history. Early prenatal care is associated with better outcomes.

Question 7

Can my due date change during pregnancy?

Accepted Answer

Yes, your due date may be adjusted based on early ultrasound findings. If the ultrasound-measured gestational age differs from the LMP-based estimate by more than 5-7 days in the first trimester, your provider may adjust the due date. Later ultrasounds are less reliable for dating due to natural variation in fetal growth rates.

Due Date Calculator — When Is My Baby Due?