Estimate your due date, track weight gain, and see trimester milestones.
Open Calculator →Last reviewed: March 2026
A pregnancy calculator is a tool that estimates your due date based on the first day of your last menstrual period (LMP). It also breaks your pregnancy into three trimesters and can help you track weight gain, fetal development, and upcoming milestones.
Most pregnancy calculators use Naegele's rule, which adds 280 days (40 weeks) to the first day of your LMP. This assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are longer or shorter, the estimate might be off by a few days.
I used a pregnancy calculator throughout my pregnancy, and it helped me understand what was happening week by week. The due date gave me a target to work toward, and the trimester breakdowns helped me prepare for each phase.
There are two main methods for calculating due dates:
This is the most common method. You take the first day of your last menstrual period, subtract three months, and add seven days. For example, if your LMP was January 1, your due date would be October 8.
This method assumes you have a 28-day cycle and ovulated on day 14. If your cycles are irregular, this estimate might not be accurate.
Early ultrasounds (before 14 weeks) can measure the crown-rump length of the embryo and estimate gestational age. If your ultrasound due date differs from your LMP due date by more than 7 days, your doctor will likely use the ultrasound date.
In my case, the ultrasound moved my due date back by 5 days because I ovulated later than expected. Don't get too attached to the original date — it's an estimate, not a guarantee.
Only about 4% of babies arrive on their exact due date. Most babies arrive within a week or two before or after the estimated date. First-time moms tend to go a bit longer (average 40 weeks and 3 days), while second-time moms average around 40 weeks and 1 day.
Think of your due date as a window, not a deadline. I spent the last two weeks of my pregnancy checking for signs of labor every time I felt a twinge. My daughter arrived 3 days before her due date, which felt like an eternity of anticipation.
If you go past 41 weeks, your doctor will likely discuss induction. But in most cases, your body knows when it's time.
Weight gain during pregnancy varies by trimester and depends on your pre-pregnancy BMI. Here's a general breakdown:
The baby itself accounts for about 7-8 pounds. The rest is amniotic fluid, placenta, increased blood volume, breast tissue, fat stores, and uterine growth.
The Institute of Medicine recommends the following total weight gain based on pre-pregnancy BMI:
These are guidelines, not rules. Your doctor will monitor your weight gain and adjust recommendations based on your health, the baby's growth, and any complications.
I gained 32 pounds with my first pregnancy (within the normal range) and 28 pounds with my second. Both babies were healthy, and I lost the weight gradually over 6-8 months postpartum.
The first trimester is when the embryo develops from a cluster of cells into a recognizable human form. By week 12, all major organs have formed, and the risk of miscarriage drops significantly.
Common symptoms include morning sickness (which can happen at any time of day), fatigue, breast tenderness, and frequent urination. Some women feel great; others feel like they've been hit by a truck.
I was exhausted during the first trimester. I slept 10 hours at night and still took naps. The fatigue lifted suddenly around week 14, and I felt like myself again.
Weight gain in the first trimester is minimal. Focus on taking a prenatal vitamin with folic acid, staying hydrated, and eating small, frequent meals if nausea is an issue.
The second trimester is often called the "golden period." Morning sickness usually fades, energy returns, and you start to show. Most women feel more comfortable than in the first or third trimester.
Milestones include feeling the baby move (quickening, usually around 18-20 weeks for first-time moms), the anatomy scan at 20 weeks, and finding out the baby's sex if you want to.
Weight gain picks up in the second trimester — about 1 pound per week. Your appetite may increase, but focus on nutrient-dense foods rather than eating for two (you only need about 300 extra calories per day).
I started walking 30 minutes a day in the second trimester and found it helped with energy, mood, and sleep. Talk to your doctor before starting any exercise routine.
The third trimester is when the baby gains most of their birth weight. Your uterus expands significantly, and you may experience back pain, shortness of breath, and Braxton Hicks contractions.
Weight gain continues at about 1 pound per week but slows down near the end. Some women even lose a pound or two in the last week as the body prepares for labor.
Common discomforts include heartburn, swollen feet and hands, difficulty sleeping, and frequent urination (the baby is pressing on your bladder). A pregnancy pillow can help with sleep.
I started counting kick counts at 28 weeks as recommended by my doctor. If the baby moved less than 10 times in 2 hours, I'd call. It never came to that, but it gave me peace of mind.
Use a pregnancy calculator or app to track your week-by-week progress. Most apps show the baby's size (compared to fruit), upcoming milestones, and symptoms you might experience.
Track your weight gain at each prenatal visit. Your doctor will plot it on a chart and let you know if you're on track.
Keep a list of questions for each prenatal visit. I forgot half my questions as soon as I walked in, so writing them down helped.
Take photos throughout your pregnancy. You might not feel like documenting the changes day by day, but looking back at the progression is worth it.
Call your doctor immediately if you experience:
When in doubt, call. Your doctor would rather get a false alarm than miss something serious.
A pregnancy calculator is a helpful tool for estimating your due date and tracking your progress, but remember that every pregnancy is different. The due date is an estimate, weight gain recommendations are guidelines, and milestones are averages.
Trust your body, listen to your doctor, and don't compare your pregnancy to someone else's. What matters most is that you and your baby are healthy.