Calculating your due date
The first thing most people do after getting a positive pregnancy test result is look up a due date calculator. That’s how ingrained the idea of a due date is in our culture. We set up our prenatal care, schedule our parental leave from work, and announce our upcoming birth with this one date. We plan our lives around this date for months and months. But what if I told you it’s just a guess?
Like many doulas, I generally refer to my clients’ due dates as “guess dates” or “estimated due dates”. I work with the understanding that babies choose their birthdays most of the time, and that there are many factors that play a role in the length of someone’s gestation. As such, I go “on-call” at 37 weeks (3 weeks before the guess date) and stay on call until the baby is born. On occasion, I’m called to a birth before the 37 week mark, and sometimes I’m on-call for more than a month before the baby comes. It’s hard to pinpoint when a baby will come.
There are many ways to calculate your EDD (estimated due date), let’s start with the most common.
Neagele’s Rule for due dates
Naegele’s rule was popularized by a German obstetrician in the 19th century and is by far the most common way to calculate baby’s due date. To use Naegele’s rule, you calculate starting on the first day of your last menstrual cycle. Yep, you’re technically “pregnant” before you actually conceive, and the first two-ish weeks of pregnancy are before you even ovulate! Isn’t pregnancy math fun?
It’s about to get even more fun! From the first day of your last cycle, simply subtract three months and add one year and seven days. Don’t worry, you can also plug it into a due date calculator online to simplify matters.
This is also something that might effect your eventual birth date, especially if you don’t have a 28 day cycle. This method assumes ovulation on day 14, implantation before day 18, and a positive pregnancy test as early as day 21. For many of us, this simply isn’t accurate, and this is where Neagele’s rule starts to break down.
Ovulation based due dates
I have been taught extensively about the menstrual cycle and have tracked my own cycles fairly regularly over the past six years. In that time, the earliest I ovulated was day 19 of my cycle, with my average being closer to day 23. That’s almost ten days different from what many due date calculators assume, which could impact so much! Good providers will take a detailed history and adjust accordingly, but some doctors stubbornly hold onto the forty week model that Neagele was working with over two hundred years ago.
The best providers will listen to their patients and adjust the EDD to 38 weeks after ovulation. This can bump the due date by over a week and make a huge difference in Baby’s lung and brain development at birth, especially since many providers prefer to induce by 41 weeks gestation. They cite concerns of going too far over the guess date. Some even encourage routine induction at 39 weeks! When guess dates are NOT individualized and are calculated using just the first day of your last period, it can be off by weeks. This can impact not only the baby’s health at birth, but also how effective an induction will be.
Early term ultrasounds
Some people don’t have regular cycles and don’t track their temperature and cervical mucus daily. Some people have never had regular cycles, others become less regular after using birth control or being pregnant previously. With my second pregnancy, I had only had a few periods after having my first child. Breastfeeding made my cycles very unpredictable. My original due date, calculated using Neagele’s rule, was October 8th. After meeting with my midwife and admitting that my cycles were irregular, she encouraged a “dating ultrasound” to reassess my due date.
Dating ultrasounds take a few key measurements, including femur length, to help calculate what Baby’s gestation is, which helps pin down an estimated due date. These scans are most accurate in the first trimester and can give a more accurate due date. With my own pregnancy, my due date was moved out over a week beyond our original guess.
After the birth
If the due date or gestational age of a newborn is in question after the birth, providers may assess a baby for gestational maturity. They judge physical and neuromuscular signs on a point system that spans from 0-50. Premature babies have lower scores, those born later have higher scores.
The physical assessment looks for things like how much lanugo (tiny hairs that babies develop in the womb to protect their skin) a baby has, the appearance and number of plantar creases (wrinkles on the soles of the baby’s feet), and the appearance of the baby’s ears, eyes, and even genitals. The neuromuscular assessment looks at how the baby moves at their joints and the baby’s posture in general.
So what does it all mean?
I often joke that due dates are useless, but that isn’t entirely accurate. It would be more accurate to say that due dates don’t indicate when Baby will actually make their appearance. Only 5% of babies are born on their estimated due date, and I have been at the live births of babies from 28-42 weeks. So when I’m planning my life around my clients, I don’t worry too much when due dates are close together. In fact, the births that came the closest together (31 hours apart) were due five weeks apart!
Really though, the goal is to stay pregnant until at least 37 weeks and hopefully give birth by about 42 weeks. Every day of pregnancy can make a difference, and getting to term is easier on the entire family (well, the person who is pregnant may not feel that way by the end, but that’s what massage is for!)
I love to encourage people to use vague terms when it comes to their guess dates. Even Catherine, Princess of Wales, only shared that she was due “in the spring” with the media. If it’s good enough for a princess, it’s good enough for us! This takes away some of the pressure that increases from well meaning and excited loved ones toward the end of pregnancy, and can help erase the idea that a due date is a deadline. Many first time parents go past their guess dates, and they should know that they are perfectly normal.
