Pregnancy Week By Week: What Happens At Each Stage

How Pregnancy Weeks Are Counted (Naegele's Rule)

Due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Conception typically occurs around day 14 of the cycle, so you are already "2 weeks pregnant" at the moment of conception.

Example: LMP January 1, 2026 → due date October 8, 2026. On March 1, 2026, you would be "8 weeks pregnant" — 6 weeks since conception.

First Trimester (Weeks 1–13)

Weeks 1–4: Fertilisation and implantation. Most people don't yet know they are pregnant. The embryo implants in the uterine lining around day 21–24.

Weeks 5–8: Heart begins beating (week 6). Major organs form. Morning sickness commonly begins (despite the name, often afternoon or evening). Breast tenderness, fatigue, and frequent urination are typical.

Weeks 9–13: The fetus is now called a fetus (previously embryo). Most early miscarriage risk passes by week 12. First-trimester screening typically offered between weeks 11–14: nuchal translucency scan and blood tests to assess Down syndrome risk.

Second Trimester (Weeks 14–27)

Weeks 14–17: Energy returns for many; morning sickness often eases. Bump may start showing. Fetal movement perceived by the pregnant person (called "quickening") usually begins between weeks 16–22.

Weeks 18–22: Anatomy scan (often called the "20-week scan") offered to all pregnancies. Checks fetal development, placenta position, amniotic fluid levels, and can reveal sex if parents wish.

Weeks 23–27: Fetus could survive outside the womb with intensive neonatal care from week 24 (the "viability line"). Glucose tolerance test for gestational diabetes typically offered at 24–28 weeks.

Third Trimester (Weeks 28–40)

Weeks 28–32: Rapid weight gain for the fetus. Braxton Hicks contractions ("practice contractions") may begin. Prenatal visits increase to fortnightly in most healthcare systems.

Weeks 33–36: Fetus settles into head-down position (usually). Group B strep test offered in US; in UK, not routinely tested but available if risk factors present.

Weeks 37–40: "Full term" begins at week 37. Labor can start any time from here. Appointments become weekly. Position of baby, dilation of cervix, and any signs of labor are monitored.

Weeks 40–42: "Late term" (week 41) and "post term" (week 42+). Induction typically offered if labor hasn't started by 41–42 weeks, due to rising risk after this point.

Due Date Accuracy

Only about 5% of babies arrive on their calculated due date. The typical range:

• Weeks 37–39: "Full term early"
• Weeks 39–40: "Full term"
• Weeks 40–41: "Full term late"
• Week 41+: "Late term"

First pregnancies average 41 weeks 1 day. Subsequent pregnancies average slightly earlier. An 8-week window around the due date is entirely normal.

Key Prenatal Appointments (US + UK)

US schedule (typical):

• Weeks 8–10: first visit, dating ultrasound, blood panel
• Weeks 11–14: first trimester screening
• Weeks 16–20: anatomy ultrasound
• Weeks 24–28: gestational diabetes screening
• Weeks 28+: fortnightly visits
• Weeks 36+: weekly visits

UK NHS schedule:

• Weeks 8–10: booking appointment with midwife
• Weeks 11–14: dating scan + screening
• Week 16: midwife appointment
• Week 20: anatomy scan
• Weeks 25, 28, 31, 34, 36, 38, 40, 41: midwife appointments

Signs Of Early Labor

In the weeks before labor:

• Lightening: baby drops lower, relieving pressure on diaphragm
• Nesting instinct: sudden urge to clean and organise
• Mucus plug / "show": discharge of cervical mucus, sometimes blood-tinged
• Braxton Hicks intensification: practice contractions may become stronger

True labor signs:

• Regular contractions that intensify over time (not eased by rest or hydration)
• Waters breaking (not everyone — happens in only about 15% before contractions)
• Lower back pain that doesn't shift with position
• Contractions 5 minutes apart for an hour → time to call midwife or hospital

Nutrition Calories During Pregnancy

Pregnancy does not "eat for two":

• First trimester: no additional calories needed
• Second trimester: ~340 extra calories/day
• Third trimester: ~450 extra calories/day

Breastfeeding after birth: 450–500 additional calories/day. Quality matters more than quantity — adequate folate, iron, calcium, and omega-3 are all critical.

Weight Gain Expectations

Institute of Medicine guidelines for total pregnancy weight gain:

• Underweight BMI (<18.5): 12.5–18 kg (28–40 lb)
• Healthy BMI (18.5–24.9): 11.5–16 kg (25–35 lb)
• Overweight BMI (25–29.9): 7–11.5 kg (15–25 lb)
• Obese BMI (30+): 5–9 kg (11–20 lb)

Most weight gain occurs in the second and third trimesters. First-trimester gain of more than 2–3 kg is unusual.

The Bottom Line

Every pregnancy is slightly different, but the broad week-by-week progression is predictable. Use the 40-week framework to understand where you are and what's typical, but trust your healthcare provider for anything specific to your pregnancy. This guide is educational and should not replace professional prenatal care.