Calculate Your Due Date
When Did I Conceive?
Calculate your likely conception date from your due date or last period.
How Many Weeks Pregnant Am I?
How Big Is My Baby?
Discover your baby's size compared to everyday items.
Baby Size by Week
Pregnancy Weight Gain Calculator
Check your recommended weight gain based on NHS and NICE guidelines.
NHS Weight Gain Guidelines
Underweight
BMI under 18.5
Recommended gain
Healthy Weight
BMI 18.5 - 24.9
Recommended gain
Overweight
BMI 25 - 29.9
Recommended gain
Obese
BMI 30+
Recommended gain
Important Note
These are general guidelines based on NHS and NICE recommendations. Every pregnancy is different. Your midwife will monitor your weight and provide personalised advice at your antenatal appointments.
First Trimester
Baby's organs begin forming. Morning sickness and fatigue common. Dating scan at 10-14 weeks.
Second Trimester
Baby grows rapidly. Feel movements from week 18-20. Anomaly scan at 18-21 weeks.
Third Trimester
Baby continues growing. More frequent check-ups. Prepare for birth and pack hospital bag.
NHS Antenatal Appointments Schedule
| Week | Appointment Type | What Happens |
|---|---|---|
| 8-10 | Booking Appointment | Medical history, blood tests, urine test, discuss birth options |
| 10-14 | Dating Scan | Confirm due date, check for multiple pregnancies, nuchal translucency screening |
| 16 | Antenatal Check | Blood pressure, urine test, discuss screening results |
| 18-21 | Anomaly Scan | Detailed scan checking baby's development, spine, heart, brain |
| 25 | Antenatal Check (first baby) | Blood pressure, urine, measure bump, discuss results |
| 28 | Antenatal Check | Blood tests, anti-D injection if Rh negative, blood pressure |
| 31 | Antenatal Check (first baby) | Blood pressure, urine, measure bump |
| 34 | Antenatal Check | Discuss birth plan, blood pressure, second anti-D if needed |
| 36 | Antenatal Check | Check baby's position, blood pressure, discuss birth options |
| 38 | Antenatal Check | Blood pressure, urine, discuss signs of labour |
| 40 | Due Date Check | Discuss options if baby hasn't arrived, membrane sweep offered |
| 41 | Post-dates Check | Membrane sweep, discuss induction options |
How Is Your Due Date Calculated?
Your estimated due date (EDD) is calculated using Naegele's Rule, the same method used by the NHS and midwives across the UK. Here's how it works:
Naegele's Rule Formula
Due Date = LMP + 280 days (40 weeks)
Or: LMP + 1 year - 3 months + 7 days
For example, if your LMP was 1st January 2025, your due date would be 8th October 2025.
Why 40 Weeks?
Pregnancy is dated from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later during ovulation. This means when you're "4 weeks pregnant," your baby is actually about 2 weeks old. The full 40 weeks accounts for:
- Weeks 1-2: Before conception (menstrual period and pre-ovulation)
- Weeks 3-4: Fertilisation and implantation
- Weeks 5-40: Embryonic and fetal development
Cycle Length Adjustment
The standard 40-week calculation assumes a 28-day cycle. If your cycle is longer or shorter, we adjust accordingly:
- Shorter cycle (e.g., 24 days): Ovulation occurs earlier, so due date moves earlier
- Longer cycle (e.g., 35 days): Ovulation occurs later, so due date moves later
- Adjustment formula: Add or subtract the difference from 28 days
How Accurate Is This Due Date?
Only about 5% of babies are born on their exact due date. Most babies arrive within a window:
| Timing | Percentage of Births | Classification |
|---|---|---|
| Before 37 weeks | ~8% | Preterm |
| 37-38 weeks | ~15% | Early term |
| 39-40 weeks | ~55% | Full term |
| 41-42 weeks | ~20% | Late term |
| After 42 weeks | ~2% | Post-term |
NHS Dating Scan
Your NHS dating scan between 10-14 weeks provides the most accurate due date, based on measuring your baby's crown-rump length (CRL). This measurement has an accuracy of ±5 days. If your scan date differs significantly from your LMP calculation, the scan date is usually used.
Frequently Asked Questions
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's Rule and is the standard method used by the NHS. If your cycle is longer or shorter than 28 days, the calculation is adjusted accordingly.
Your dating scan measures your baby's crown-rump length (CRL) to estimate gestational age. This is more accurate than LMP calculations because it doesn't rely on knowing when you ovulated. If the scan date differs by more than 5-7 days from the LMP date, your midwife will likely use the scan date as your official EDD.
According to NHS guidelines, full term pregnancy is between 39-40 weeks. Early term is 37-38 weeks, late term is 41-42 weeks, and post-term is after 42 weeks. About 55% of babies are born during the full term window of 39-40 weeks.
Contact your midwife or maternity unit if you experience: regular contractions, your waters break, bleeding, reduced baby movements, severe headaches, vision changes, sudden swelling, or anything that concerns you. If you're past 37 weeks with regular contractions 5 minutes apart, it's time to contact the hospital.
The NHS will offer a membrane sweep at 40-41 weeks to help start labour naturally. If labour hasn't started by 41+0 to 42+0 weeks, you'll be offered induction of labour. Approximately 20% of babies are born after 41 weeks, and most are perfectly healthy.
Due date calculators provide an estimate based on average pregnancy length. Only 5% of babies arrive on their exact due date. The calculation is most accurate when you have regular cycles and know exactly when your last period started. Your dating scan provides a more personalised estimate.
Yes, if you know your conception date, add 266 days (38 weeks) to calculate your due date. This is because the standard 40-week calculation includes 2 weeks before conception. IVF patients often know their exact conception date from the embryo transfer.
If your periods are irregular, LMP-based calculations may be less accurate. Your dating scan becomes especially important for determining your due date. Tell your midwife about your irregular cycles so they can factor this into your care plan.
Medical Disclaimer
This calculator provides estimates only and should not replace professional medical advice. Always consult your midwife, GP, or healthcare provider for personalised guidance about your pregnancy. If you have any concerns about your pregnancy, contact your midwife or maternity unit immediately.