Pregnancy Calculator
Pregnancy
Due Date Calculator
Enter your last period date and instantly get your due date, how many weeks pregnant you are, your trimester, and a complete week-by-week development timeline. Based on NHS and OB/GYN guidelines.
Pregnancy Calculator -- Find Your Due Date, Week, and Trimester
Finding out you are pregnant is one of life's most exciting moments -- and one of the first questions every expectant mother has is: When is my due date? Our pregnancy calculator gives you an instant, accurate answer based on the same method your doctor uses. Whether you are in the very first days of pregnancy or counting down the final weeks, this tool gives you everything you need to understand and track your pregnancy journey.
How Is a Pregnancy Due Date Calculated?
Your estimated due date (EDD) is calculated using Naegele's Rule, named after German obstetrician Franz Karl Naegele. The formula is elegantly simple: take the first day of your last menstrual period, add 280 days (or 40 weeks), and the result is your estimated due date.
The standard calculation assumes a 28-day menstrual cycle and ovulation at day 14. If your cycle is shorter or longer than 28 days, our calculator automatically adjusts your due date accordingly -- adding or subtracting the difference from the standard cycle length. This makes our calculator significantly more accurate than basic tools that assume a fixed 28-day cycle for everyone.
It is important to understand that this is an estimated due date. Only about 5% of babies are born on their exact due date. The majority arrive within two weeks before or after. Your doctor will confirm your due date with an ultrasound scan at your 8-12 week appointment, which can measure fetal size to provide an even more precise date.
How Many Weeks Pregnant Am I?
Pregnancy is measured in weeks from the first day of your last menstrual period -- not from the date of conception. This means that technically, you are considered "pregnant" for the first two weeks before conception actually occurs. This can seem counterintuitive, but it is the universal standard used by healthcare providers globally.
To find how many weeks pregnant you are, our calculator counts the number of days between the first day of your last period and today, then divides by seven. The result includes both weeks and days -- for example, "8 weeks and 4 days" -- which is exactly how your midwife or OB/GYN will refer to your pregnancy at each appointment.
Knowing your exact week of pregnancy is important because many prenatal tests, scans, and appointments are scheduled based on specific weeks. For example, the first trimester screening test is performed between weeks 10 and 13, and the detailed anatomy scan takes place at weeks 18 to 20. Use our Ovulation Calculator to understand your cycle better.
The Three Trimesters of Pregnancy
First Trimester -- Weeks 1 to 12
The first trimester is the most critical period of fetal development. During weeks 1 to 12, your baby develops from a single fertilised egg into a fetus with a beating heart, developing brain, forming organs, and recognisable human features. By the end of the first trimester, your baby is approximately 7 centimetres long and weighs about 14 grams.
Most women experience morning sickness, fatigue, breast tenderness, and frequent urination during the first trimester. Miscarriage risk is highest during this period, which is why many couples wait until week 12 -- when the risk drops significantly -- before sharing their news. Key tests during the first trimester include blood type confirmation, STI screening, and first trimester screening for chromosomal conditions. Monitor your Blood Pressure throughout pregnancy as hypertension can develop.
Second Trimester -- Weeks 13 to 26
The second trimester is widely considered the most enjoyable phase of pregnancy. Morning sickness typically subsides, energy levels return, and the pregnancy begins to show with a visible baby bump. Your baby grows rapidly -- from 7 cm at week 13 to approximately 35 cm by week 26. The baby begins to develop fingerprints, can hear sounds from outside the womb, and starts to move.
The anatomy scan ultrasound -- performed between weeks 18 and 20 -- checks all of the baby's major organs and can usually reveal the baby's sex. At week 24, your baby reaches the viability milestone, meaning they could potentially survive outside the womb with intensive medical support. Nutrition becomes critically important during this trimester. Check your Iron levels as iron deficiency anaemia is common in the second trimester. Use our Pregnancy Weight Gain Calculator to track healthy weight gain.
Third Trimester -- Weeks 27 to 40
The third trimester is the final stretch of pregnancy. Your baby gains most of its weight during this period -- going from approximately 900 grams at week 27 to an average of 3.4 kg at birth. The baby's lungs mature, brain development accelerates, and the immune system strengthens. Most babies turn head-down (cephalic position) in preparation for birth during weeks 32 to 36.
Common symptoms include back pain, heartburn, shortness of breath (as the baby pushes against the diaphragm), frequent urination, swollen ankles, and difficulty sleeping. Doctor visits become weekly from week 36. Ensure adequate Calcium intake as your baby is rapidly mineralising its bones. At week 37, your pregnancy is considered full-term. Most babies arrive between weeks 38 and 41. After 42 weeks, induction is usually recommended.
Critical Pregnancy Milestones by Week
| Week | Milestone | What Happens |
|---|---|---|
| Week 4 | Positive test possible | hCG levels detectable; embryo implants in uterus |
| Week 6-8 | First heartbeat | Heart begins beating; first ultrasound often scheduled |
| Week 10-13 | First trimester screening | Nuchal translucency scan; Down syndrome blood test |
| Week 12 | End of first trimester | Miscarriage risk drops; baby fully formed |
| Week 16-20 | Baby movements felt | First kicks -- called quickening |
| Week 18-20 | Anatomy scan | Detailed ultrasound; gender reveal possible |
| Week 24 | Viability milestone | Baby can potentially survive outside womb |
| Week 28 | Third trimester begins | Glucose tolerance test; Rhesus injection if needed |
| Week 36 | Weekly visits begin | Group B Strep test; baby moves into position |
| Week 37 | Full term | Baby ready to be born at any time |
| Week 40 | Due date | Estimated date of delivery (EDD) |
| Week 42 | Post-term | Induction usually recommended |
Essential Nutrients During Pregnancy
Pregnancy dramatically increases your nutritional needs. The most critical nutrients for a healthy pregnancy include:
- Folic acid (folate): 400-600 mcg daily before conception and during the first 12 weeks prevents neural tube defects. Found in leafy greens, beans, and fortified foods.
- Iron: 27 mg daily (nearly double the non-pregnant requirement) to support increased blood volume and prevent anaemia. Use our Iron Intake Calculator to check your needs. Vitamin C from our Vitamin C Calculator dramatically improves iron absorption.
- Calcium: 1000-1300 mg daily for baby's bone and tooth development. If dietary intake is insufficient, your body will leach calcium from your own bones. Check with our Calcium Calculator.
- Vitamin D: 600-800 IU daily supports calcium absorption and immune function. Most pregnant women are deficient -- assess with our Vitamin D Calculator.
- Omega-3 DHA: 200-300 mg daily for fetal brain and eye development. Found in oily fish, flaxseed, and supplements.
- Iodine: 220 mcg daily for thyroid function and baby's brain development.
- Water: 2.3-3 litres daily -- hydration needs increase significantly in pregnancy. Use our Water Intake Calculator.
For pregnancy-friendly recipes packed with these essential nutrients, visit our Daily Health Kitchen for iron-rich meals, folate-packed dishes, and calcium-boosting snacks for every trimester.
Pregnancy Weight Gain Guidelines
Healthy pregnancy weight gain depends on your pre-pregnancy BMI. The recommended ranges are: underweight (BMI under 18.5): 13-18 kg; normal weight (BMI 18.5-24.9): 11-16 kg; overweight (BMI 25-29.9): 7-11 kg; obese (BMI 30+): 5-9 kg. Use our Pregnancy Weight Gain Calculator for your personalised target, and track your weight at each appointment. Gaining too little can restrict fetal growth; gaining too much increases risk of gestational diabetes and complications.
What to Expect After Birth
Once your baby arrives, our Breast Milk Storage Calculator will help you safely store expressed milk. Track your newborn's growth with our Baby Weight Percentile Calculator to ensure your baby is growing as expected. When you are ready, our Ovulation Calculator will help you understand your fertility cycle again. Predict when your period might return with our Period Calculator.
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