When Will Baby Arrive? How Due Date Calculators Work
One of the very first questions every expectant parent has is “When will my baby arrive?” When you’re staring down at that positive pregnancy test, finding out your estimated due date is an exciting step.
As a midwife, I often get asked how due dates are calculated and how accurate they really are. So let’s have a little chat about how due date calculators work and what they really mean for your baby’s grand entrance.
What Is a Due Date Calculator?
A due date calculator is a simple tool used to estimate your baby’s arrival date. It works by calculating 40 weeks from the first day of your last menstrual period (LMP). That’s because, in most cases, we can’t pinpoint the exact day of conception, but we can usually work out when your last period started.
You’ll often hear this called your estimated due date or EDD. It’s important to remember it’s just that – an estimate.
Why Do We Count from the Last Period?
You might be wondering, “Why do we count from my last period and not from when I actually got pregnant?” Great question. Pregnancy is typically dated from the first day of your last period, not the day of conception, because ovulation and conception usually happen about two weeks later – and that can be tricky to know for sure.
By using the first day of your LMP, we’ve got a consistent starting point, even if your cycle isn’t perfectly regular.
What If My Cycles Are Irregular?
If your cycles are longer or shorter than the average 28 days, the due date from your LMP might be slightly off. That’s where your dating scan comes in.
How Accurate Is the Due Date?
The due date gives us a rough idea of when baby might arrive. But only about 4% of babies are born on their actual due date!
According to NICE guidelines, most babies arrive somewhere between 37 and 42 weeks. That means your baby might come a bit early or fashionably late, and both are completely normal.
What About the Dating Scan?
Your dating scan, usually offered between 10 and 14 weeks of pregnancy (as per NHS guidance), gives a more accurate due date than LMP alone. It measures your baby from head to bottom – called the crown-rump length – to estimate how far along you are.
If the scan shows a different date than the one calculated from your LMP, we usually go with the scan result, especially if the difference is more than five days.
Can Anything Predict When Labour Will Actually Start?
I wish I could say yes, but the truth is, there’s no guaranteed way to predict the day labour will begin. Every pregnancy is unique. Things like your baby’s position, your body’s readiness, and even your stress levels can all play a part.
You’ll hear people say things like “first babies are always late” – but that’s not always the case. Some arrive early, some right on time, and others take their sweet time.
What If I Go Over My Due Date?
If your baby hasn’t arrived by 41 weeks, your midwife will talk to you about your options. Many maternity units offer membrane sweeps and discuss induction of labour around 41 to 42 weeks. This will depend on your preferences and any individual circumstances.
