Twin Pregnancy Calculator — Week 18: Di/Di, Mo/Di & Mo/Mo
You are 18 weeks pregnant with twins. Twin gestation deviates meaningfully from singleton pregnancy from week 18 onwards — chorionicity, growth discordance and delivery timing all change. Here is the exact week-18 twin picture.
Twin Pregnancy Calculator
Enter the first day of your last menstrual period (LMP). EDD = LMP + 280 days.
Baby at week 18
Per twin, week 18 development: Ears reach final position on head. Uterus grows rapidly. Meconium accumulates in bowel.
Mother's body — what changes
Twin pregnancy at week 18 differs — expect: Baby movements more consistent. Braxton-Hicks contractions may start (painless tightening).
Nutrition & supplements
Add ~340 kcal/day above pre-pregnancy baseline. Prioritise iron (27 mg/day), calcium (1000 mg), omega-3 DHA (200–300 mg) and 71 g protein. Continue prenatal vitamin. For twin gestation add an extra 300 kcal on top (total ~600 second trimester, ~750 third), 50% more iron, and split hydration to 3–3.5 L/day.
Safe exercise
Continue 150 min/week. Avoid supine (flat-on-back) exercises after week 20 — they compress the vena cava. Swimming and prenatal Pilates are ideal now. For twins reduce intensity 20% earlier — bed rest is not routinely indicated but modified activity from week 24 is prudent.
Doctor visit checklist — week 18
- ✓Anatomy ultrasound (18–22 weeks) — check all organs and placenta location
- ✓Chorionicity confirmed at 10–14 weeks (Di/Di, Mo/Di, or Mo/Mo)
- ✓Growth scans every 4 weeks (Di/Di) or every 2 weeks (Mo/Di, TTTS surveillance)
- ✓Maternal-fetal medicine (MFM) referral by week 20
Warning signs — call your provider immediately
- ⚠Heavy bleeding (soaking a pad in an hour)
- ⚠Severe abdominal pain that does not ease
- ⚠Fever above 38°C / 100.4°F
- ⚠Sudden gush of fluid (early rupture of membranes)
- ⚠Absent fetal movement for 12+ hours after 24 weeks
- ⚠Severe headache + visual disturbance (preeclampsia)
Week 18 — Frequently asked questions
How accurate is the Twin Pregnancy Calculator at week 18?
The calculator uses standard gestational-age arithmetic (LMP + 280 days for standard; egg retrieval + 266 days for IVF day-5). Ultrasound crown-rump length between weeks 8–13 is more accurate (±5 days) than LMP — adjust EDD accordingly if the scan differs by more than 7 days.
What size is my baby at week 18?
Approximately the size of a bell pepper — length 14.2 cm and weight 190 g per twin (combined weight roughly 1.8× singleton for well-nourished mothers). These are median values from SMFM, ACOG twin guidelines and NICE multiple pregnancy references.
Which symptoms are normal in week 18?
Baby movements more consistent. Braxton-Hicks contractions may start (painless tightening). Every pregnancy is different — call your provider if anything on the warning-signs list appears.
Should I have an ultrasound at week 18?
Anatomy ultrasound (18–22 weeks) — check all organs and placenta location. Routine scan windows are: dating (10–13w), anatomy (18–22w), growth (28–32w).
How much weight should I gain by week 18?
Approximately 4.2 kg cumulative if you started at a normal BMI (IOM guideline). Underweight mothers gain more; overweight mothers less. Twin gestation targets 16–24 kg total (higher than singleton 11–16 kg).
Is my twin pregnancy Di/Di, Mo/Di or Mo/Mo?
Chorionicity is confirmed on the 10–14 week ultrasound by looking at the membrane between twins — 'lambda sign' = Di/Di (safest), 'T sign' = Mo/Di (requires TTTS monitoring every 2 weeks), no membrane = Mo/Mo (highest risk, delivery by 34 weeks).
When are twins usually delivered?
Di/Di twins: 38 weeks. Mo/Di twins: 36–37 weeks. Mo/Mo twins: 32–34 weeks by planned C-section. About 60% of twins deliver preterm (<37 weeks) spontaneously.
Content aligned with SMFM, ACOG twin guidelines and NICE multiple pregnancy. Reviewed by Dr. Maria Rehman, MD (OB/GYN). This page is educational and does not replace personal medical advice — call your obstetrician for any concern.