You finally confirm you are pregnant. Congratulations! One of the first things to pop in your head: the calculation of your pregnancy due date.
Most pregnancies last around 38 weeks after conception or 40 weeks. The best way to determine your estimated delivery date (EDD), or your due date, is to count 40 weeks or 280 days from the first day of your last menstrual period (LMP). But it's not as easy as it sounds.
Many women barely notice any early signs of pregnancy, which are similar and can be confused with premenstrual symptoms. Often, women take an at-home pregnancy test kit only when her period is late. If you're trying to conceive, the best time to take a pregnancy test to get more accurate results is when your period has already been delayed for a week or two. By that time, you might have already breezed through Week 1 through Week 8 of your pregnancy.
How to calculate for your due date
It's impossible to know the day you actually conceived or when your partner's sperm fertilized your egg, so you cannot pinpoint your delivery date accurately. Doctors have two ways to arrive at a close estimate: based on your LMP and early ultrasound.
Your due date based on early ultrasound
An ultrasound can be a good predictor of your due date, as it does so based on the size of the developing fetus, though not all pregnant women have it done. An early ultrasound is not as reliable in predicting due dates of older pregnant women and those who have a history of miscarriage and pregnancy complications.
Your due date based on your LMP
There's an easier way to compute for your due date based on your LMP. Obstetrician-gynecologist and sonologist Dr. Maria Carla Esquivias-Chua of the MCEC Mother and Child Ob-Gyn Ultrasound and Pedia Clinic, in Kamuning, Quezon City, calls it the Naegele's Rule.
Instead of counting 40 weeks or 280 days forward from the first day of a woman's LMP, doctors subtract three months and seven days from her LMP date. For example, if your LMP is March 20, your expected date of delivery (EDD) is December 27. It's still an estimate but on target.
If you have a regular period, the EDD based on LMP should be close to your EDD based on your ultrasound. Still, Dr. Esquivias-Chua explains a pregnant woman's EDD is not set in stone but somewhat flexible. A pregnant woman can go into labor three weeks before her EDD or two weeks after. First-time pregnant women typically go into labor nearer their due date.
Your due date based on your estimated conception date
Calculating the EDD based on a woman's first day of her LMP does not apply to women who have irregular periods. Without an ultrasound, you might be able to recall the date when you think you conceived and add 266 days to arrive at your estimated due date. It's not as reliable as the first two methods, but it's a start to gauge how far along you are in your pregnancy on your first prenatal checkup.
Other tools that can estimate your delivery date
If you can't pinpoint when you conceived or remember the first day of your LMP and have no access to ultrasound technology, there are still other ways to help figure out how far along you are in your pregnancy. An EDD is vital to assist your doctor in care for you and your baby.
A doppler will let you and your doctor hear your baby's heartbeat, which is usually heard around Week 6 to Week 10 of the pregnancy. A pregnant woman's account of feeling the baby in her womb move, which generally happens between Week 18 to Week 22, can also be a gauge. While these pregnancy milestones can give clues when you can expect your baby to arrive, they may vary from one pregnant woman to another.
Going to your prenatal checkup regularly and monitoring the size of your uterus via an internal (vaginal) examination (IE) and measuring the size of your baby bump or your fundal height (the length from your pubic bone to the top of your uterus) can also help your health practitioner confirm your due date.
Can you plan your due date? Not exactly, even with the help of some smartphone apps, or unless your doctor eventually opts for an elective C-section for health and safety reasons. Still, if you have a regular period, you can aim for a specific month for when you want your baby to arrive.