How Do Doctors Work Out Your Due Date?


When will my baby be born? Is it based on your last period, an ultrasound, or a date given to you by your doctor? How do you know when to prepare for delivery if all three are on separate days? Aren’t these reasonable questions? I’m sure there are hundreds of pregnant women out there who believed they had their pregnancy due date all figured out. They went to their initial doctor’s visit, and the doctor rescheduled them. This blog by Nurturey will help you understand how doctors calculate your due date.

If you had regular periods before becoming pregnant, your doctor would calculate your due date using the date of your most recent menstrual cycle. This stems from the fact that to conceive, your body ovulated—or released an egg—roughly in the middle of your cycle, which was then fertilised by sperm. So that was the point at which you became pregnant.

The baby has been growing for two weeks by the time most women miss a period and discover they’re pregnant. However, the mother is genuinely four weeks along because the gestational period begins on the first day of your previous menstruation.

What Factors Go into Determining My Pregnancy Due Date?

The first day of your last menstruation is usually 280 days (40 weeks or around ten months — also known as ten lunar months) from your due date. However, your pregnancy due date may differ from the “280-day norm” if your cycles are irregular or not 28 days apart. Therefore, your doctor may order an ultrasound to identify your due date precisely.

Please inform your health care practitioner if you are positive of your conception date (the date you became pregnant). This information can assist you in establishing your expected delivery date (EDD). Because a full-term pregnancy can last anywhere from 37 to 40 weeks and six days, your actual delivery date may differ from your estimated date of delivery, also known as an estimated date of confinement or EDC. Only a tiny percentage of babies are born on their due dates. Only about 5% of women give birth on their pregnancy due date.

To be clear again, the 40-week gestational period begins on the first day of your last period, which adds two weeks to the gestational period when your kid wasn’t even born…

Is an ultrasound a better approach to determine my pregnancy due date? 

Ultrasound is frequently performed to determine how far you have irregular cycles before becoming pregnant. Because all foetuses regularly grow during the first trimester and early second, an ultrasound is the most accurate way to date a pregnancy.

In other words, no matter when your last period is, if your baby measures nine weeks two days when you have an ultrasound, that’s how far along you are. Some women with regular cycles are perplexed as to why their ultrasound due date does not correspond to the end of their last menstrual period. Ovulation isn’t a precise science, and it can occur earlier or later than expected, causing your pregnancy due date to change somewhat.

That’s fine…a few days or perhaps a week’s delay won’t affect your plans. Instead, your doctor will use the due date from your ultrasound as a guide. 

What happens if my kid becomes too big or too small later? 

Is it necessary to change my due date, or will I be able to deliver sooner? After 20 weeks, your doctor will measure your belly at every prenatal check-up. In centimetres, that measurement should correspond to your gestational age. The doctor may arrange an ultrasound if you measure smaller or larger than you should be. This will inform them whether the difference is due to the baby’s actual size, the amount of fluid surrounding the baby, or just the manner you’re carrying the baby. If the infant is genuinely smaller or larger than they should be, underlying concerns may drive the growing disparity. For example, moms with uncontrolled diabetes (Type 1, Type 2, or Gestational) give birth to exceptionally large children.

When a baby is little, it could be due to a placenta not functioning correctly. These inconsistencies may or may not have an impact on delivery. For example, if your baby’s measurements are continuously little and below what they should be, your doctor may decide that you should be delivered since your baby would be safer outside than inside.

If your baby is particularly large and you’re measuring 40 weeks at 37 weeks, your body may believe it’s done and induce labour early…. or you may wait until your pregnancy due date, and everything will go according to plan. If you have any concerns about your pregnancy due date, speak with your doctor, who will be able to provide you with the most accurate information.


Your healthcare provider will be able to more correctly monitor your baby’s growth if you know when your baby is due. Because specific laboratory tests alter throughout your pregnancy, knowing your exact pregnancy due date will help your healthcare provider keep track of these tests and manage preterm labour if it arises.

As soon as you learn you’re pregnant, see a midwife or a doctor. This is so that they can schedule your prenatal (antenatal) care and ensure that you receive all of the information and support you require to have a safe pregnancy. As the primary supplier of the Digital Personal Child Health Record (DPCHR) program, Nurturey’s Pink Book is a digital upgrade to the NHS paper red book. It gives you instant access to your child’s medical records as well as trustworthy NHS advice. In addition, you can monitor your child’s growth, celebrate their milestones and keep a record of your prenatal tests and pregnancy timeline using Nurturey’s intuitive tools.


About Author

Garima Capoor is a doctor of medicine by profession, who stumbled her way into content writing, much to her parents' bemusement. Research is her favourite word and she uses it generously while trying to understand the fascinating dynamics of parents and children. With no children of her own, her niece and nephew are the guinea pigs for everything she learns (the family dog was off limits).

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