Baby Size Sports Ball worksheet

1. Closest ball by weight: A duckpin bowling ball

1. A due date is 280 days after the first day of the last period, so count back 280 days to find that starting day

   $\text{Mar 4, 2026}\text{−}280\text{\hspace{0.17em}days}$$=\text{May 28, 2025}$

2. Count the days from the first day of the last period to the counting-from date

   $\text{Jan 6, 2026}\text{−}\text{May 28, 2025}$$=223\text{\hspace{0.17em}days}$

3. Split the days into whole weeks and leftover days, since pregnancy is counted in weeks

   $223\text{÷ 7}$$=\text{31 weeks, 6 days}$

4. Put week 31 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}31-0.00354\text{×}31\text{²}$$=7.4681$

5. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{7.4681}$$=\mathrm{1,751}\text{\hspace{0.17em}g}$

6. Find the ball nearest in weight, a duckpin bowling ball. Divide the baby's typical weight by its weight to compare them

   $\frac{\mathrm{1,751}\text{\hspace{0.17em}g}}{\mathrm{1,701}\text{\hspace{0.17em}g}}$$=1.03$

7. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1.03\text{is within 1.5 times either way →}\text{A duckpin bowling ball}$

2. Closest ball by weight: A size 7 basketball

1. Put week 24 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}24-0.00354\text{×}24\text{²}$$=6.507$

2. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{6.507}$$=670\text{\hspace{0.17em}g}$

3. Find the ball nearest in weight, a size 7 basketball. Divide the baby's typical weight by its weight to compare them

   $\frac{670\text{\hspace{0.17em}g}}{600\text{\hspace{0.17em}g}}$$=1.12$

4. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1.12\text{is within 1.5 times either way →}\text{A size 7 basketball}$

3. Closest ball by weight: A baseball

1. Put week 16 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}16-0.00354\text{×}16\text{²}$$=4.9838$

2. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{4.9838}$$=146\text{\hspace{0.17em}g}$

3. Find the ball nearest in weight, a baseball. Divide the baby's typical weight by its weight to compare them

   $\frac{146\text{\hspace{0.17em}g}}{145.3\text{\hspace{0.17em}g}}$$=1$

4. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1\text{is within 1.5 times either way →}\text{A baseball}$

4. Closest ball by weight: A 12-inch softball

1. A due date is 280 days after the first day of the last period, so count back 280 days to find that starting day

   $\text{Oct 20, 2025}\text{−}280\text{\hspace{0.17em}days}$$=\text{Jan 13, 2025}$

2. Count the days from the first day of the last period to the counting-from date

   $\text{May 13, 2025}\text{−}\text{Jan 13, 2025}$$=120\text{\hspace{0.17em}days}$

3. Split the days into whole weeks and leftover days, since pregnancy is counted in weeks

   $120\text{÷ 7}$$=\text{17 weeks, 1 day}$

4. Put week 17 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}17-0.00354\text{×}17\text{²}$$=5.1989$

5. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{5.1989}$$=181\text{\hspace{0.17em}g}$

6. Find the ball nearest in weight, a 12-inch softball. Divide the baby's typical weight by its weight to compare them

   $\frac{181\text{\hspace{0.17em}g}}{188.2\text{\hspace{0.17em}g}}$$=0.96$

7. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $0.96\text{is within 1.5 times either way →}\text{A 12-inch softball}$

5. Closest ball by weight: A duckpin bowling ball

1. Put week 32 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}32-0.00354\text{×}32\text{²}$$=7.577$

2. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{7.577}$$=\mathrm{1,953}\text{\hspace{0.17em}g}$

3. Find the ball nearest in weight, a duckpin bowling ball. Divide the baby's typical weight by its weight to compare them

   $\frac{\mathrm{1,953}\text{\hspace{0.17em}g}}{\mathrm{1,701}\text{\hspace{0.17em}g}}$$=1.15$

4. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1.15\text{is within 1.5 times either way →}\text{A duckpin bowling ball}$

6. Closest ball by weight: A volleyball

1. Put week 20 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}20-0.00354\text{×}20\text{²}$$=5.802$

2. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{5.802}$$=331\text{\hspace{0.17em}g}$

3. Find the ball nearest in weight, a volleyball. Divide the baby's typical weight by its weight to compare them

   $\frac{331\text{\hspace{0.17em}g}}{270\text{\hspace{0.17em}g}}$$=1.23$

4. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1.23\text{is within 1.5 times either way →}\text{A volleyball}$

7. Closest ball by weight: A bocce pallino (the small target ball)

1. Count the days from the first day of the last period to the counting-from date

   $\text{Apr 12, 2025}\text{−}\text{Jan 11, 2025}$$=91\text{\hspace{0.17em}days}$

2. Split the days into whole weeks and leftover days, since pregnancy is counted in weeks

   $91\text{÷ 7}$$=\text{13 weeks, 0 days}$

3. Put week 13 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}13-0.00354\text{×}13\text{²}$$=4.2957$

4. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{4.2957}$$=73\text{\hspace{0.17em}g}$

5. Find the ball nearest in weight, a bocce pallino (the small target ball). Divide the baby's typical weight by its weight to compare them

   $\frac{73\text{\hspace{0.17em}g}}{90\text{\hspace{0.17em}g}}$$=0.81$

6. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $0.81\text{is within 1.5 times either way →}\text{A bocce pallino (the small target ball)}$

8. Closest ball by weight: A tennis ball

1. Put week 12 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}12-0.00354\text{×}12\text{²}$$=4.0522$

2. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{4.0522}$$=58\text{\hspace{0.17em}g}$

3. Find the ball nearest in weight, a tennis ball. Divide the baby's typical weight by its weight to compare them

   $\frac{58\text{\hspace{0.17em}g}}{57.7\text{\hspace{0.17em}g}}$$=1.01$

4. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $1.01\text{is within 1.5 times either way →}\text{A tennis ball}$

9. Closest ball by weight: An 8-pound bowling ball

1. A due date is 280 days after the first day of the last period, so count back 280 days to find that starting day

   $\text{Jan 2, 2027}\text{−}280\text{\hspace{0.17em}days}$$=\text{Mar 28, 2026}$

2. Count the days from the first day of the last period to the counting-from date

   $\text{Dec 28, 2026}\text{−}\text{Mar 28, 2026}$$=275\text{\hspace{0.17em}days}$

3. Split the days into whole weeks and leftover days, since pregnancy is counted in weeks

   $275\text{÷ 7}$$=\text{39 weeks, 2 days}$

4. Put week 39 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}39-0.00354\text{×}39\text{²}$$=8.1417$

5. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{8.1417}$$=\mathrm{3,435}\text{\hspace{0.17em}g}$

6. Find the ball nearest in weight, an 8-pound bowling ball. Divide the baby's typical weight by its weight to compare them

   $\frac{\mathrm{3,435}\text{\hspace{0.17em}g}}{\mathrm{3,628.7}\text{\hspace{0.17em}g}}$$=0.95$

7. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $0.95\text{is within 1.5 times either way →}\text{An 8-pound bowling ball}$

10. Closest ball by weight: A golf ball

1. A due date is 280 days after the first day of the last period, so count back 280 days to find that starting day

   $\text{Oct 23, 2025}\text{−}280\text{\hspace{0.17em}days}$$=\text{Jan 16, 2025}$

2. Count the days from the first day of the last period to the counting-from date

   $\text{Apr 5, 2025}\text{−}\text{Jan 16, 2025}$$=79\text{\hspace{0.17em}days}$

3. Split the days into whole weeks and leftover days, since pregnancy is counted in weeks

   $79\text{÷ 7}$$=\text{11 weeks, 2 days}$

4. Put week 11 into the Hadlock formula for typical weight. The answer is the natural log (ln) of the weight in grams

   $0.578\text{+}0.332\text{×}11-0.00354\text{×}11\text{²}$$=3.8017$

5. Undo the log by raising e (about 2.718) to that power, then round to the nearest gram

   $e^{3.8017}$$=45\text{\hspace{0.17em}g}$

6. Find the ball nearest in weight, a golf ball. Divide the baby's typical weight by its weight to compare them

   $\frac{45\text{\hspace{0.17em}g}}{45.93\text{\hspace{0.17em}g}}$$=0.98$

7. The ratio is between 0.67 and 1.5, so the two weigh about the same. That makes it a fair match

   $0.98\text{is within 1.5 times either way →}\text{A golf ball}$